I do struggle with speed, tending to speed up. I'll look into metronomes. I will also try using a strap, but does anyone have suggestions for holding a ukulele? I have the same issue of an arm getting tired. At the moment I lean on an armrest or against a table while playing to deal with it. Another method I've found is to sit with one leg crossed and angled upward.

Practicing every single day has made a huge difference in my playing -- I heartily recommend it, even if you only do it for a few minutes.