This has been bugging me as IMO it is something Adobe could fix...
When I go to 'Auto Sync' a whole album with say radial filter it takes forever. However, if I change the settings on one image and then select all other images and click 'Sync' it happens much faster. This is similar with actions. The combination of more than a few actions really slows the sync down massively where as if you sync and select the same selections it is much faster.
Can anyone explain to me why this is? Auto Sync seems buggy in this respect.
I don't recall other reports of those symptoms, but I might have missed them. I recommend that you post a bug report on the official Adobe feedback forum: Lightroom Classic CC | Photoshop Family Customer Community . Adobe product developers read everything posted there and actively participate but are rarely seen in this forum. Be sure to include the first ten lines or so of the menu command Help > System Info. Even better would be a screen recording showing the comparative behaviors (a picture, a thousand words, etc.).