There are a few posts around that describe how to quickly jump to the end of a long thread. Start here, and see if it helps.
The quick instruction is: it depends on the platform you’re using. On a desktop/laptop you should see a smaller secondary scroll bar along the right side of the thread. Grab that and drag to the bottom.
On a mobile device, find the post counter at lower right of the screen, tap that to open a similar scroll bar, drag it down to the bottom to get to the last post (or anywhere you want in the thread).
Once you’ve done this for a thread, the next time you visit it will automatically jump to the last post you read.