One more thing I noticed/checked and I believe may be of interest, again not in particular for this proposed Knuth multiplier change, but in general. Some filters have much weaker performance for mixed lookup. To be honest I have expected that for some(branch misprediction) but I do not know enough about all filters to say if slowdowns I have observed are as expected. Interesting that some fast_ have bad performance for mixed lookup, although naively I would assume SIMD code is branchless. I presume the issue is that sometimes we operate on more than 256 bits so there is branching... What I find interesting beside this slowdown is that unordered_flat_set also has bad performance for mixed lookup so bloom filters that do not degrade on mixed lookup look much better than if we just compare 100%/0% cases. For example if we round performance to 1 decimal digit and compare unordered_flat set 4.7/3.1/*9.4* vs filter<int, 1ul, block<unsigned long, 7ul>, 1ul, hash<int>, allocator<int>, mcg_and_fastrange> 3.3/3.3/*3.3* comparison looks much much different if we drop mixed performance(bolded part). Here is output my small test run for 1M items, I have also attached source if somebody wants to run it on his machine, it is modified benchmark code. Note that I did not add mixed numbers to html output, they are printed on cerr for cases when slowdown is detected. usual disclaimer: my machine, only 1 compiler, etc. Slow mixed lookup unordered_flat_set_filter<int> FPR 0 insertion_time 17.5341 successful_lookup_time 4.68546 unsuccessful_lookup_time 3.10828 mixed_lookup_time 9.3794 Slow mixed lookup filter<int, 1ul, fast_multiblock32<11ul>, 0ul, hash<int>, allocator<int>, mcg_and_fastrange> FPR 0.1169 insertion_time 3.13323 successful_lookup_time 3.11569 unsuccessful_lookup_time 2.1553 mixed_lookup_time 9.8229 Slow mixed lookup filter<int, 1ul, fast_multiblock32<13ul>, 0ul, hash<int>, allocator<int>, mcg_and_fastrange> FPR 0.028 insertion_time 3.35237 successful_lookup_time 3.57547 unsuccessful_lookup_time 2.49646 mixed_lookup_time 10.173 Slow mixed lookup filter<int, 1ul, fast_multiblock32<11ul>, 0ul, hash<int>, allocator<int>, fastrange_and_fixed_mcg> FPR 0.1201 insertion_time 3.19758 successful_lookup_time 3.23826 unsuccessful_lookup_time 2.24502 mixed_lookup_time 9.93281 Slow mixed lookup filter<int, 1ul, fast_multiblock32<13ul>, 0ul, hash<int>, allocator<int>, fastrange_and_fixed_mcg> FPR 0.0319 insertion_time 3.32857 successful_lookup_time 3.33717 unsuccessful_lookup_time 2.60953 mixed_lookup_time 10.5385 Slow mixed lookup filter<int, 1ul, fast_multiblock32<11ul>, 1ul, hash<int>, allocator<int>, mcg_and_fastrange> FPR 0.084 insertion_time 3.33082 successful_lookup_time 3.55582 unsuccessful_lookup_time 2.28511 mixed_lookup_time 9.61687 Slow mixed lookup filter<int, 1ul, fast_multiblock64<11ul>, 0ul, hash<int>, allocator<int>, mcg_and_fastrange> FPR 0.0761 insertion_time 4.61749 successful_lookup_time 4.8703 unsuccessful_lookup_time 3.57926 mixed_lookup_time 11.4899 Slow mixed lookup filter<int, 1ul, fast_multiblock64<11ul>, 1ul, hash<int>, allocator<int>, mcg_and_fastrange> FPR 0.0629 insertion_time 4.47389 successful_lookup_time 4.73559 unsuccessful_lookup_time 3.62872 mixed_lookup_time 11.0488 Slow mixed lookup filter<int, 1ul, fast_multiblock32<13ul>, 1ul, hash<int>, allocator<int>, mcg_and_fastrange> FPR 0.0183 insertion_time 3.49621 successful_lookup_time 3.31442 unsuccessful_lookup_time 2.77045 mixed_lookup_time 9.50943 Slow mixed lookup filter<int, 1ul, fast_multiblock64<13ul>, 0ul, hash<int>, allocator<int>, mcg_and_fastrange> FPR 0.0154 insertion_time 5.32834 successful_lookup_time 6.67898 unsuccessful_lookup_time 4.46843 mixed_lookup_time 12.1819 Slow mixed lookup filter<int, 1ul, fast_multiblock64<14ul>, 1ul, hash<int>, allocator<int>, mcg_and_fastrange> FPR 0.0121 insertion_time 6.06002 successful_lookup_time 6.80858 unsuccessful_lookup_time 4.30832 mixed_lookup_time 12.2018 Slow mixed lookup filter<int, 1ul, fast_multiblock32<11ul>, 1ul, hash<int>, allocator<int>, fastrange_and_fixed_mcg> FPR 0.0914 insertion_time 3.14643 successful_lookup_time 3.08782 unsuccessful_lookup_time 2.14325 mixed_lookup_time 9.40797 Slow mixed lookup filter<int, 1ul, fast_multiblock64<11ul>, 0ul, hash<int>, allocator<int>, fastrange_and_fixed_mcg> FPR 0.0757 insertion_time 4.24599 successful_lookup_time 4.46389 unsuccessful_lookup_time 3.33413 mixed_lookup_time 11.2218 Slow mixed lookup filter<int, 1ul, fast_multiblock64<11ul>, 1ul, hash<int>, allocator<int>, fastrange_and_fixed_mcg> FPR 0.0638 insertion_time 4.29631 successful_lookup_time 4.46474 unsuccessful_lookup_time 3.33839 mixed_lookup_time 11.0688 Slow mixed lookup filter<int, 1ul, fast_multiblock32<13ul>, 1ul, hash<int>, allocator<int>, fastrange_and_fixed_mcg> FPR 0.0191 insertion_time 3.28589 successful_lookup_time 3.28611 unsuccessful_lookup_time 2.50493 mixed_lookup_time 9.61269 Slow mixed lookup filter<int, 1ul, fast_multiblock64<13ul>, 0ul, hash<int>, allocator<int>, fastrange_and_fixed_mcg> FPR 0.0145 insertion_time 5.298 successful_lookup_time 5.89132 unsuccessful_lookup_time 5.36365 mixed_lookup_time 15.9068 Slow mixed lookup filter<int, 1ul, fast_multiblock64<14ul>, 1ul, hash<int>, allocator<int>, fastrange_and_fixed_mcg> FPR 0.0125 insertion_time 6.79997 successful_lookup_time 6.54907 unsuccessful_lookup_time 5.04036 mixed_lookup_time 13.1906