The hot water pressure is fine in both bathrooms but low in the kitchen... Why??

Did you check the screen filter on the end of the faucet as it may have a bunch of junk plugging it up and that would account for low pressure. Also some of the european faucets have internal filters that you can't get to without pulling it all apart
I just last week finished having my pipes redone because of a leak under the cement foundation where we can't get to so we had to run it all through the walls and ceiling what a mess hope yours is just a screened filter problem

I don't suggest pulling a european faucet apart unless you have an rebuild kit with the O-ring kit as the O-rings tend to tear or break when you pull it apart (sorry should have said that in first post)

So this problem could be caused by a leak... GREAT! I guess I get to crawl under the house.
Thanks for the information.. I chuckled when you talked about the "european faucet ".. My kitchen faucet is as old as I am...

If the faucet is that old it may have deterated inside and may need to be replaced they do sell kits to fix some of the inner parts but may be just as easy to replace
Good luck