After talking with a user on a relatively slow 2G connection about performance (https://www.mediawiki.org/wiki/Thread:Project:Support_desk/Offline_editing), he suggested:

I think they should integrate a test like these in their testing framework.
Its almost an obligatory test for whoever wants make their service accessible
from the whole world.

I'm not sure how feasible this is, but it does seem like it fits the goals for the Foundation.

Version: 1.22.0
Severity: enhancement