Preview is so slow

The slowness to start preview was mentioned in the annoucement of MFR preview (https://community.adobe.com/t5/after-effects-beta/multi-frame-rendering-is-here-aka-the-multithreading-you-ve-been-asking-for/td-p/11885358/page/10) - we are working on making it start instantly, it just requires a few more weeks of coding work to finish it up.

I can confirm this issue from my side. We were able to replicate it on 2 very different systems regardless of the computer specs and using multiple test projects, including the benchmark project Adobe shared on here. 
Specs below reflect my (fairly low end) system but it's the same on a 6 core computer with 64 Gb of RAM

Steps to reproduce:

1. open benchmark project for multi-frame rendering
2. render (part) of the timeline to ram - either using speculative preview or by triggering preview

press space to start playback from ram
 
Result:

noticable (5s) delay between pressing the space button and the audio playback starting

video playback even more delayed and starting very slow

video takes another 7s to catch up to realtime speed

Expected: instant playback in real time

Note:

Disabling Multi-Frame Rendering in the preferences speeds up the ram playback responsiveness to normal levels (instant start at real time speed) but slows down the rendering.

 
System info
    Application: After Effects (Beta) v22.0.0.32
    OS: Windows v10.0.19042, RAM: 23.84 GB, CPUs (logical): 8

you can solve this in the beta version, in Preferences -> Memory and Performance -> and disable the mult-render.
the lag should stop.

I have a pretty decent hardware tho 
R9 3900X 12 Cores - 24 Threads | RTX 3070 8GB | 32 GB RAM | 3500MB Read / 3000MB Write NVMe SSD