Ok so after a full test with my 4k 24fps 30sec clip of Avengers infinity war (HEVC) we are with :

SVT-AV1 1.4.1 no grain denoise off: 19:18 minutes / 0.82 fps / 9.2% of source

SVT-AV1 1.4.1 grain synthesis 20 denoise off: 20:34 / 0.76fps avg / 9.2% of source ( -6.05% speed )

SVT-AV1 1.5 no grain denoise off: 15:20 minutes / 1.18fps avg / 9.356% of source ( +20.75% speed )

SVT-AV1 1.5 grain synthesis 20 denoise off: 16:09 / 1.09fps avg / 9.381 of source ( +16.11% of speed )

SVT-AV1 1.6 no grain denoise off: 10:57 minutes / 1.43fps avg / 9.168% of source ( +44.89% speed )

SVT-AV1 1.6 grain synthesis 20 denoise off: 11:43 / 1.33fps /9.192% of source ( +40.41% speed )

which means 20% speed boost between svt-av1 1.4.1 and 1.5. And another 30% between 1.5 and the new 1.6.

GG devs, you are amazing!

( this is with preset 3 crf 18 at 4k and the settings for grain when used, all in handbrake. mind that this is with a Ryzen 5800X3D with a little bit of downvolting for perf )