I am using solely my wrist to tremolo pick, and at high speeds it is pretty loose and relaxed. But when I slow it down, my picking hand seems to get a bit tense. I'm not sure why. I think it's one of those mental blocks. I've practiced playing tremolo picking at high speeds for so long that it seems to be almost harder to slow it down.

Do I simply need to practice at slow speeds just as much as I do at high speeds to be comfortable with both? Why does this happen to me? And it's only with tremolo picking. If I'm picking 3nps scales, sweeping, legato, tapping, it seems fine. But slowing down my tremolo picking feels weird.
Try using other joints to produce the motion, like your knuckles.

Sometimes the only thing I'm moving is the fingers themselves in a really tight circle.
"Virtually no one who is taught Relativity continues to read the Bible."

I've noticed that people tend to have two speeds they can play at. Their comfort zone, and their wankery. The wankery is when you are just blazin' as fast as you can, you don't have complete control, but it's passable. The issue is just as you describe: you can't slow it down. In all of my experience, this is normal. Just keep working until your 'comfort zone' playing is at that level, as it is a more realistic meter of your playing level.
Quote by nightwind
You must never double the leading tone ever. Failure to comply will result in a fugue related death.
No, that's not really the case. My "wankery" is in complete control and my picking hand is extremely relaxed. I accent on every measure and can continue on and on forever.

Although my slow picking has been getting better lately. I think it's probably just that I haven't practice slowed picking nearly as much as I have fast picking. Meh.