Thanks for the feedback—it is true that Tab Stash currently doesn't handle thousands of OPEN tabs (in the same window) very well. However, it is known to handle large sets (tens of thousands) of STASHED tabs quite well, with minimal memory usage, and I've put quite a lot of work into making sure it stays that way.

The 500MB of RAM you're referencing is likely because you have thousands of tabs loaded in the UI from when you first stashed your open tabs—this will normally be much smaller because UI elements are created only when they need to be shown.

As for the one-by-one processing of tabs, that's unfortunately a limitation in Firefox's bookmarks API. Having lots of parallel bookmark operations running can mess up Firefox Sync and cause other problems in the bookmarks database.