#1
slow practice definitely helps, but do i have to practice everything slow? for example, i recently learned the main riff to washington is next, it came fairly easily and after an 15 minutes or so i could play it at speed. if something comes easily should i still practice slowly? obviously i should for solos and other things that are of a more difficult nature but what about the rest of it? is there any benefit to practicing something you already play well slowly?
#2
If you can already play something up to speed flawlessly, there's no need to practice playing it slowly to get better at playing it; you can already play it.

Now, playing something you know how to play fast at a slow speed? That's good for a warm-up and will help you keep rhythm at the front of your mind. It's alright if you can play it fine up to speed, but if you can't play it slow and in time, that's something to practice on. If you can, good, keep it up.