I have mid 2012 retina MBP with 8 Gig ram and SSD. I downgraded from High Sierra to Sierra due to performance problems. I was very happy with Sierra in terms of performance.
Upgraded to Mojave Public Beta last week and am very happy with the performance. Equally as fast as Sierra if not slightly faster. No lags and everything generally feels snappy.
Just my tuppence worth.
EDIT: first three days, my hard drive was constantly working as Dropbox and Photos synced everything up and then it settled down quite nicely.