At the moment listenbrainz seems to be somewhat challenged to cope with all requests and each page submission is taking about 20 seconds to complete.

Currently I am importing my list as well and after ~50 completed pages I started noticing my browser getting noticeably slower.

Apparently the importer creates as many requests as possible and in my case the browser already has ~700 outstanding requests (to send) with only 100 pages completed.

The importer should be adjusted to only request a few LAST.FM pages in a advance to what is already completed.