The alloc reduction is significant and depends on number of constructors and the number of constructor parameters (e.g. 4.6x reduction for 3 constructors).

The CPU case depends on whether[ActivatorUtilitiesConstructor] is used. If used, the perf improvements are much better:

When used, CPU improvement of ~2x depending on number of constructor parameters.
When not used, CPU improvement of ~10%.

The optimizations include:

Leveraging the recent changes inActivatorUtilities.CreateInstance() should respect [ActivatorUtilitiesConstructor] #99175 for when[ActivatorUtilitiesConstructor] is used. We can now ignore most processing of other constructors.
Leveraging theConstructorInvoker class added in v8; this allows use ofSpan<object> over stack-allocated objects (if <= 4 parameters) or the ability to re-use an allocation ofobject[] over varying number of constructor parameters.

Benchmarks

| Method                         | Job        | Toolchain              | Mean      | Error    | StdDev   | Median    | Min       | Max       | Ratio | RatioSD | Gen0   | Allocated | Alloc Ratio ||------------------------------- |----------- |----------------------- |----------:|---------:|---------:|----------:|----------:|----------:|------:|--------:|-------:|----------:|------------:|| CreateInstance_1               | Job-ZNAFCX | \DI_AFTER\corerun.exe  |  52.41 ns | 0.979 ns | 0.916 ns |  52.14 ns |  51.04 ns |  54.11 ns |  1.00 |    0.00 | 0.0022 |      24 B |        1.00 || CreateInstance_1               | Job-OPANVX | \DI_BEFORE\corerun.exe |  56.80 ns | 0.789 ns | 0.659 ns |  57.04 ns |  55.88 ns |  58.31 ns |  1.08 |    0.02 | 0.0075 |      80 B |        3.33 ||                                |            |                        |           |          |          |           |           |           |       |         |        |           |             || CreateInstance_3               | Job-ZNAFCX | \DI_AFTER\corerun.exe  | 113.72 ns | 1.604 ns | 1.500 ns | 113.75 ns | 111.31 ns | 116.46 ns |  1.00 |    0.00 | 0.0037 |      40 B |        1.00 || CreateInstance_3               | Job-OPANVX | \DI_BEFORE\corerun.exe | 133.66 ns | 2.790 ns | 3.213 ns | 133.19 ns | 129.45 ns | 139.60 ns |  1.18 |    0.03 | 0.0173 |     184 B |        4.60 ||                                |            |                        |           |          |          |           |           |           |       |         |        |           |             || CreateInstance_5               | Job-ZNAFCX | \DI_AFTER\corerun.exe  | 247.52 ns | 3.243 ns | 2.708 ns | 246.91 ns | 242.39 ns | 250.86 ns |  1.00 |    0.00 | 0.0150 |     160 B |        1.00 || CreateInstance_5               | Job-OPANVX | \DI_BEFORE\corerun.exe | 255.55 ns | 4.273 ns | 3.788 ns | 255.48 ns | 247.91 ns | 263.24 ns |  1.03 |    0.02 | 0.0298 |     320 B |        2.00 ||                                |            |                        |           |          |          |           |           |           |       |         |        |           |             || CreateInstance_1_WithAttrFirst | Job-ZNAFCX | \DI_AFTER\corerun.exe  |  41.69 ns | 0.652 ns | 0.610 ns |  41.52 ns |  40.96 ns |  42.76 ns |  1.00 |    0.00 | 0.0022 |      24 B |        1.00 || CreateInstance_1_WithAttrFirst | Job-OPANVX | \DI_BEFORE\corerun.exe |  57.47 ns | 1.305 ns | 1.503 ns |  57.00 ns |  55.08 ns |  59.93 ns |  1.39 |    0.04 | 0.0074 |      80 B |        3.33 ||                                |            |                        |           |          |          |           |           |           |       |         |        |           |             || CreateInstance_3_WithAttrFirst | Job-ZNAFCX | \DI_AFTER\corerun.exe  |  66.32 ns | 1.323 ns | 1.105 ns |  66.13 ns |  64.98 ns |  69.18 ns |  1.00 |    0.00 | 0.0036 |      40 B |        1.00 || CreateInstance_3_WithAttrFirst | Job-OPANVX | \DI_BEFORE\corerun.exe | 135.23 ns | 2.654 ns | 2.607 ns | 134.25 ns | 132.12 ns | 140.97 ns |  2.04 |    0.05 | 0.0174 |     184 B |        4.60 ||                                |            |                        |           |          |          |           |           |           |       |         |        |           |             || CreateInstance_5_WithAttrFirst | Job-ZNAFCX | \DI_AFTER\corerun.exe  | 125.68 ns | 2.449 ns | 2.515 ns | 125.37 ns | 121.91 ns | 129.73 ns |  1.00 |    0.00 | 0.0149 |     160 B |        1.00 || CreateInstance_5_WithAttrFirst | Job-OPANVX | \DI_BEFORE\corerun.exe | 262.42 ns | 3.933 ns | 4.208 ns | 263.44 ns | 254.65 ns | 268.10 ns |  2.09 |    0.05 | 0.0300 |     320 B |        2.00 ||                                |            |                        |           |          |          |           |           |           |       |         |        |           |             || CreateInstance_1_WithAttrLast  | Job-ZNAFCX | \DI_AFTER\corerun.exe  |  41.54 ns | 0.456 ns | 0.405 ns |  41.53 ns |  40.74 ns |  42.25 ns |  1.00 |    0.00 | 0.0022 |      24 B |        1.00 || CreateInstance_1_WithAttrLast  | Job-OPANVX | \DI_BEFORE\corerun.exe |  57.71 ns | 1.284 ns | 1.479 ns |  57.26 ns |  55.75 ns |  60.65 ns |  1.40 |    0.04 | 0.0076 |      80 B |        3.33 ||                                |            |                        |           |          |          |           |           |           |       |         |        |           |             || CreateInstance_3_WithAttrLast  | Job-ZNAFCX | \DI_AFTER\corerun.exe  |  65.44 ns | 0.840 ns | 0.744 ns |  65.45 ns |  64.58 ns |  66.73 ns |  1.00 |    0.00 | 0.0038 |      40 B |        1.00 || CreateInstance_3_WithAttrLast  | Job-OPANVX | \DI_BEFORE\corerun.exe | 133.59 ns | 2.429 ns | 2.272 ns | 133.87 ns | 129.38 ns | 136.90 ns |  2.04 |    0.04 | 0.0176 |     184 B |        4.60 ||                                |            |                        |           |          |          |           |           |           |       |         |        |           |             || CreateInstance_5_WithAttrLast  | Job-ZNAFCX | \DI_AFTER\corerun.exe  | 125.32 ns | 5.427 ns | 6.249 ns | 122.53 ns | 119.54 ns | 138.34 ns |  1.00 |    0.00 | 0.0149 |     160 B |        1.00 || CreateInstance_5_WithAttrLast  | Job-OPANVX | \DI_BEFORE\corerun.exe | 257.39 ns | 5.177 ns | 5.754 ns | 257.49 ns | 249.35 ns | 271.52 ns |  2.06 |    0.10 | 0.0303 |     320 B |        2.00 |

.NET Framework is also slightly improved for both CPU and allocs:

.NET Framework 4.8.1 Benchmarks

BEFORE| Method                         | Mean     | Error     | StdDev    | Median   | Min      | Max       | Gen0   | Allocated ||------------------------------- |---------:|----------:|----------:|---------:|---------:|----------:|-------:|----------:|| CreateInstance_1               | 1.391 us | 0.0269 us | 0.0309 us | 1.385 us | 1.355 us |  1.447 us | 0.0650 |     417 B || CreateInstance_3               | 4.355 us | 0.0778 us | 0.0690 us | 4.348 us | 4.264 us |  4.464 us | 0.1575 |    1043 B || CreateInstance_5               | 9.039 us | 0.1724 us | 0.1771 us | 8.978 us | 8.799 us |  9.430 us | 0.2889 |    1926 B || CreateInstance_1_WithAttrFirst | 1.993 us | 0.0234 us | 0.0208 us | 1.989 us | 1.957 us |  2.038 us | 0.0788 |     538 B || CreateInstance_3_WithAttrFirst | 5.031 us | 0.0971 us | 0.0909 us | 5.004 us | 4.898 us |  5.216 us | 0.1800 |    1164 B || CreateInstance_5_WithAttrFirst | 9.790 us | 0.1928 us | 0.1980 us | 9.701 us | 9.551 us | 10.255 us | 0.3104 |    2046 B || CreateInstance_1_WithAttrLast  | 2.046 us | 0.0354 us | 0.0347 us | 2.033 us | 1.996 us |  2.117 us | 0.0808 |     538 B || CreateInstance_3_WithAttrLast  | 5.405 us | 0.2172 us | 0.2414 us | 5.372 us | 5.155 us |  5.935 us | 0.1829 |    1163 B || CreateInstance_5_WithAttrLast  | 9.787 us | 0.0755 us | 0.0630 us | 9.780 us | 9.700 us |  9.877 us | 0.3179 |    2046 B |AFTER| Method                         | Mean     | Error     | StdDev    | Median   | Min      | Max      | Gen0   | Allocated ||------------------------------- |---------:|----------:|----------:|---------:|---------:|---------:|-------:|----------:|| CreateInstance_1               | 1.257 us | 0.0148 us | 0.0124 us | 1.254 us | 1.246 us | 1.290 us | 0.0601 |     393 B || CreateInstance_3               | 4.120 us | 0.0342 us | 0.0320 us | 4.124 us | 4.073 us | 4.181 us | 0.1464 |     947 B || CreateInstance_5               | 8.723 us | 0.0864 us | 0.0808 us | 8.708 us | 8.611 us | 8.867 us | 0.2405 |    1725 B || CreateInstance_1_WithAttrFirst | 1.925 us | 0.0215 us | 0.0201 us | 1.920 us | 1.904 us | 1.959 us | 0.0767 |     514 B || CreateInstance_3_WithAttrFirst | 4.909 us | 0.0628 us | 0.0588 us | 4.895 us | 4.835 us | 5.012 us | 0.1582 |    1067 B || CreateInstance_5_WithAttrFirst | 9.309 us | 0.0847 us | 0.0751 us | 9.278 us | 9.199 us | 9.470 us | 0.2623 |    1846 B || CreateInstance_1_WithAttrLast  | 1.931 us | 0.0288 us | 0.0269 us | 1.926 us | 1.898 us | 1.975 us | 0.0804 |     514 B || CreateInstance_3_WithAttrLast  | 4.885 us | 0.0635 us | 0.0594 us | 4.862 us | 4.815 us | 5.022 us | 0.1576 |    1067 B || CreateInstance_5_WithAttrLast  | 9.392 us | 0.1023 us | 0.0957 us | 9.360 us | 9.277 us | 9.575 us | 0.2593 |    1846 B |