Changing BIGINT to INT

There should not be any downside. The user ID is cast to int in many places in the WP PHP files anyway.

But I don’t think this is a real performance problem. If your queries are slow, it might be more useful to inspect each of them and look for opportunities to improve the queries or to add indexes.
If your bottleneck is really IO on the file system of your DB host, consider finding a better DB server, because that will hit you in other cases too.

This issue has been raised on the WordPress Trac 14 years ago, and it was closed by Matt, the owner project leader …

in the hope that WP may someday be successful enough to need it

So if you ever get into the situation that your blog has every second person on the planet as a member, don’t forget to send Matt a Thank You!

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