Wilburrkins:
Did you manage to find a solution as I have the same problem and it is driving me crazy. Thank you.
It can sometimes work to go to each section of the list separately though the completed section ie
https://mydramalist.com/dramalist/Wilburrkins/completed
That only works sometimes (it is fine for currently watching, but I can just go to my most recent updates for that-the 10 most recent updates are on a profile page at the lower right corner, and the widget in the settings menu [widgets] makes it easy to grab up to the last 50 updates in any section of the watchlist (in progress, completed etc); I go to the widget every few months to make sure my custom list for titles I am watching this year has everything on it since I sometimes forget to add movies&spontaneous binges to it). My completed list is very long (our total lists about the same size, just in diff sections) and still hangs loading in normal logged in windows, so I just open my whole watchlist in a private window when I want to look for something from it or my PTW (ie if someone asks for a rec) then open the links in the regular logged in one.
None of these is exactly a solution, but they are my workarounds for now (mainly the custom list I have been using to edit my progress for titles that aren't currently airing and the private tab to look at my entire watchlist and be able to sort it by rating etc when considering what to recommend to someone).
Sorry I can't be of much help... maybe 2200 titles is the magic cutoff when it starts to fail since we are both in that 100# range.
Ah, it is slightly better (bc it doesn't seem to load the images for titles like when I hold down on a title and the quick -/+ isn't there, only the edit box, at least using Firefox) on the mobile version of the site if you are on a browser that gives the option (I prefer everything in desktop mode, though, and one site won't make me switch back and forth).