Does travel at relativistic speed slow proton decay?
If they DO decay, then it would.
...As best I can say from popularly available info, (I'm no physicist by any means) they're not sure if protons actually DO decay. The theory doesn't prohibit it, but proton decay has never been observed.
So if it happens, it happens slowly, and if the proton is travelling at a rate near c, it would happen even more slowly.