Practicing slow is good to get the idea together, but I only started figuring it out properly by crashing through arpeggios and correcting the mess along the way.

I'm not sure if my technique is 'correct', but I've managed to separate all the notes mostly with fretting hand muting, but a bit of right hand palm as well for some shapes. Once I got the idea by blasting through them, I went back to slowly building them up cleanly. I reckon it might have taken me 3 months of 15 minutes a day practicing 5 string minor arps before I nailed it at speed.... I'm not sure if that's average or just me.

on your hands doing different things when you speed up to when you go slow, maybe you should increase speed in smaller increments (?) It's a bit dull I know but I'm forever telling people that "If you increase speed by 1bpm on the metronome every day, in 120 days you'll be able to do it at 208bpm" or similar....

you get the picture... :D