The only performance issues that I'm aware of in 5.8 are:
- The debugger can be a lot slower in 5.8. I think this has already been fixed in bleadperl.
- There is a new hashing algorithm which may give worse behaviour in some circumstances. In general however it should give better performance.
- The pervasive unicode support can slow things down (e.g. regexes on unicode are slower than regexes on ASCII) - but you can switch that off.
There were, of course, a lot of performance enhancements in 5.8 too.
So it comes down to what you do. Personally I've not noticed any performance difference.