Also, (when re-read a few times) the symptoms could be caused by a damp air filter.
If so, this is an easy fix, but sometimes can be missed by mechanics who haven't seen it before.
Water can drip off the middle screw of the plastic scuttle cover onto the air filter box. If the cover is slightly deformed, or was not put back properly, it can dampen the air filter. This can cause a feeling of low power until it warms up and dries out a bit.
Fix is to re-fit the air filter cover properly, and put some sealant on that middle screw.