I had this issue once, and it was a particular file that wasn’t indexing properly, so it would try to reindex every time it started. I had a very large vault, and to troubleshoot I needed to add/remove folders or batches of files to narrow down which file(s) was not indexing properly. There may be an easier way to troubleshoot, a sync log that Obsidian keeps somewhere, but once you can identify the files it isn’t indexing properly and recreate them, you should be good to go.