@Tgr it doesnt make sense that it can show results if looking for a smaller number (100) but shows nothing for a bigger number (500). this misleads users into thinking those sets of filters cannot find anything, but in reality they just need to lower the number.

on the other hand, mediasearch and search can show "partial results" when they time out, like https://commons.wikimedia.org/w/index.php?search=insource%3A%2Fthe%2F .

In T336957#8883545, @RoyZuo wrote:

on the other hand, mediasearch and search can show "partial results" when they time out, like https://commons.wikimedia.org/w/index.php?search=insource%3A%2Fthe%2F .

That's likely because their engine supports showing partial results on timeout whereas the (traditional) SQL engine used by recent changes does not.

In T336957#8883545, @RoyZuo wrote:

this misleads users into thinking those sets of filters cannot find anything

If you feel the error message is not clear enough, we can reword it.

More generally, T20228: Add pagination (offset, prev/next, until/from/to) to watchlist and recent changes in the old (non-JS) UI / T163429: Provide UI for paging through Watchlist and Recent Changes results in the JS-enhanced UI and then disabling large paging sizes is probably the most feasible way to work around RC/watchlist query performance issues.

Larger-scale changes have been discussed in T307328: Scalability issues of recentchanges table but that would require the WMF or another large organization to treat this problem as a high priority.

thx guys for all your answers.
glad to know that other users have raised similar issues.
indeed, if those other tasks are resolved, this one will be done too.