GUI Viewer dropping frames in Beta 2, not in Beta 1

After installing v20b2 I opened a large complex project that was originally created in v19.x and worked on in v20b1. But playback became sluggish with many dropped frames in the GUI viewer. At the same time playback appeared smooth and normal on my TV monitor via Decklink, and audio was playing back normally. Just the viewer couldn't keep up, like it was only able to display 10fps of a 25fps playback.

I am using a dual screen layout. This was happening on edit, colour, and fairlight pages. Same with proxies enabled or disabled. The entire timeline was fully render-cached with a continuous blue line. Forcing re-cache of clips made no difference. Media and cache are on fast thunderbolt NVMe drives.

Also weirdly the Decklink was sending 24p to the TV according to TV info, even though timeline and monitoring were set to 25p. After changing timeline's monitoring output to 30p then back to 25p it reset the Decklink to outputting 25p properly again. This however seemed to have no effect on the viewer's sluggish performance.

I was unable to figure it out. Resolve restarts and system reboots made no difference. So I rolled back to v20b1, and the problem went away - the viewer is once again performing as expected on Beta 1.

Other relevant info and system specs:
- Cloud project (internet working fine)
- 1080p25 timeline and monitoring. Timeline colour science Davinci YRGB Colour Managed, colour space Davinci Wide Gamut/Intermediate, Output colour space Rec.709/2.4
- Preferences/System/General "Use 10-bit precision in viewers" and "Use Mac display colour profiles for viewers" are both checked.
- Mac Studio M2 Ultra/24 Core CPU/60 core GPU/128GB RAM
- MacOS Sequoia 15.4.1
- Decklink Mini Monitor 4K (Desktop Video driver v14.5) via Thunderbolt to PCIe chassis > LG C2 TV ref monitor