[Boulder]

I did some tests, and in my case it looks like this:

1080p, ctu 32, F 1 - 1.55 fps - 15209.92 kbps
1080p, ctu 32, F 2 - 1.98 fps - 15160.69 kbps
1080p, ctu 32, F 3 - 2.00 fps - 15216.68 kbps
1080p, ctu 32, F 4 - 2.00 fps - 15218.20 kbps

1080p, ctu 64, F 1 - 0.96 fps - 15183.31 kbps
1080p, ctu 64, F 2 - 1.41 fps - 15195.30 kbps
1080p, ctu 64, F 3 - 1.58 fps - 15194.53 kbps
1080p, ctu 64, F 4 - 1.68 fps - 15176.67 kbps

720p, ctu 32, F 1 - 2.95 fps - 6035.36 kbps
720p, ctu 32, F 2 - 3.98 fps - 6038.02 kbps
720p, ctu 32, F 3 - 4.23 fps - 5942.17 kbps
720p, ctu 32, F 4 - 4.29 fps - 6038.01 kbps

I do use --limit-tu 0 and --limit-refs 1, otherwise pretty much close to --preset slower. The Vapoursynth part utilizes ~20-25% of CPU.