Slow practice never outlives its usefulness. Speed will benefit from the relatively frictionless motions of slow, fluid practice.
I always start out slow each day. But I can't limit myself to slow practice with the performance so close.
Have you ever considered bringing up the tempo one hand at a time? When you can go full speed or faster hands-separate, the full performance will go much better, I promise.
It's the same problem. Either one arm hurts or both arms hurt.
If I practice it hands separate, it doesn't accomplish anything but making each arm hurt separately so that I can't get any good hands together practice done, which is what I need. I have to perform it hands together, after all.
The only time I ever practice hands separate any more is if I want to work out a kink - lately, that's just the left hand parts that are black key octaves with a white key tritone in the middle (there are two measures in the piece where that occurs, and those are the hardest bits) or one measure in the 'recapitulation' at the end that sometimes gives me trouble. To practice the whole thing hands separately is just a waste of strength.