When I go to a very long thread on my iPod touch and scroll down to the point where new posts are loaded, the page freezes for a few seconds.
This seems normal. It has to load more replies past a certain scroll threshold, even on PCs.
The problem is that it freezes when that happens - as in, I can’t scroll up or tap anything.
Maybe try another browser. Your browser seems to “block” while it is waiting for the requests of the page’s JavaScript to finish.