The special page does a SELECT us_key FROM uploadstash WHERE us_user = 1 without limit and does not allow pagination. There is a index on us_user, so that is not the problem.

But for every entry of the stash the metadata are looked up in the database with SELECT us_user,us_key,us_orig_path,us_path,us_props,us_size,us_sha1,us_mime,us_media_type,us_image_width,us_image_height,us_image_bits,us_source_type,us_timestamp,us_status FROM uploadstash WHERE us_key = 'xxxxxx.tmp' LIMIT 1, maybe also some interaction with the file backend is done, which can take long time with many files

On commons the stash is stored for 48 hours, so the problem with the timeout should be go away, when the files goes away. But that does not help to resume something with the special page.

There is at least a list module in the api which could make it possible to look up the files https://commons.wikimedia.org/w/api.php?modules=query%2Bmystashedfiles, but that require some skills to request and use.

Change 1003523 had a related patch set uploaded (by Umherirrender; author: Umherirrender):

[mediawiki/core@master] specials: Use a pager on Special:UploadStash

https://gerrit.wikimedia.org/r/1003523

Change 1003523 merged by jenkins-bot:

[mediawiki/core@master] specials: Use a pager on Special:UploadStash

https://gerrit.wikimedia.org/r/1003523