After a post is clicked, it seems that all thumbnails are forced to refresh. (Attached image shown immediately after a click)
Also, what may just be the result of a cache clear, doom scrolling seems to cause a similar lag after scrolling past +50 posts. Thumbnail loading becomes laggy, but not sure if related to the issue above.
When the lag starts, Connect will lag fairly bad for as long as a minute. I have also turned off image preload, but that had no effect.
Side bug, changing the title in the new post edit cleared my first post body.
I have come across this a few times now as well on the latest beta, and as you said it seems to happen after about 50+ posts where all thumbnails just keep refreshing as you try and scroll.
I too had done a cache clear at one point before this started happening to try and address a different issue so it may be related to that.