There is a third option - you could be overtrained. I suspected this to be the case because there is no reason you should not be progressing more. I can pick 4 notes per click at 110 bpm using only downstrokes. The muscles you use when picking your fastest need time to rest after an intense speed session. I suspect you aren’t getting enough time for your hand to recover and grow stronger and faster.
I suggest trying something different. I suggest you try the following for a month. It sounds like your speed hasn’t been increasing recently so I don’t think you have much to lose and you may make some very good progress.
Work on your tremolo picking for 30 minutes but only every other day. You can still play guitar on the days you don’t work on tremolo picking but don’t play anything physically stressful.
Don’t use the metronome. You might be tempted to to clock your speed but wait until you’ve done this for a month and then go back to the metronome if you want.
As part of your tremolo picking practice pick a note on one string as fast as you can until you feel the muscles in your forearm start to burn ( that’s an accumulation of lactic acid). Keep going until you feel the burn so much that you’re actually starting to play significantly slower. Rest 30 seconds or so, no more than 60 seconds and repeat. Keep doing this repeatedly until 30 minutes have gone by. I suggest spending some time on each of the 6 strings so you get used to the feel of the different sized strings.
If at any time you feel a sharp pain, which is very different from the slow burn of lactic acid buildup, stop playing immediately, ice the area that started to hurt for 15 minutes and take the next day completely off. Then on the next day you can resume your practice.
People vary according to how much exercise stress they can endure and how much time they need to recover before they can exercise again. Some can endure a tremendous amount of stress and still make progress. Others need a longer period of time to recover between intense sessions (I think this is you). Think of this just like some people can lay outing the sun a long time before getting a sunburn while others get burned very quickly. If a person who burns easily tries to lay out in the sun a long time, he will burn, his skin will peel off, and he won’t get a tan; he’ll get blisters. This kind of person needs less exposure to the sun to make progress on his suntan.
It’s hard for me to tell if your motion is wrong since I can’t see you play. Can you video record yourself tremolo picking? There may be something wrong with your mechanics that is holding you back. One obvious one I can think is if you are just moving the pick back and forth with any type of a rotating or turning motion, then try adding a turning or rotating motion to your tremolo picking, sort of like if you were stirring a drink quickly, your wrist would orate; it wouldn’t just move back and forth. Another way to look at the motion is turning the key in your car ignition. If you don’t use any rotational movement already I suggest you incorporate it in roughly half of your tremolo picking practice to see if you start progressing more while using that motion.
Let me know how this works for you.