You have a couple of options:
- Give it Leftovers for passive recovery
- Give it a Choice Scarf to make it faster
- Teach it Sleep Talk and Rest
But overall, I really wouldn't worry about it. This is in-game we're talking about. The opposing Pokemon aren't EV/IV trained, don't have good natures, and are usually underleveled, so Magnezone will usually outspeed even though it is slow. I speak from experience- I used a Magnezone in Platinum and I outsped most of the time.