The old relationship editor has issues with submitting lots of relationships for huge releases at once because it tried to include too many (all?) edits into one request to the server. (I was barely able to edit ~1000 track releases with it in the past as the submission of hundreds of new relationships resulted in lots of timeouts and there were even memory leaks, see MBS-10982.)

In order to make submission work reliably, the new relationship editor submits each edit with its own request. Due to rate limiting this is much slower, you can watch either the UI highlights of pending edits disappear or the network requests in the browser console with a frequency of about one or two edits per second. I had already tested the new relationship editor with a few ~100 track releases for which I created new works, assigned them and added a composer credit for each (so about 300 edits in total). The relationship editor took about 3 minutes to submit but it worked flawlessly. (I haven't tested yet with one of these ~1000 track releases as I haven't come across one that needs to be edited and the test server is too slow to handle so many requests.)

So I would suggest to find a middle ground between all edits and one edit per request