In terms of arithmetic and other primitive operations, I'm sure Parrot will trounce the P5VM. This will be helpful in coding loops and testing limits and so on, the nuts and bolts of what holds a program together.
But individual features may get bogged down. For example, a while back I mused that Perl 6 Patterns would be much slower than Perl 5 Regex's because you can't tell it not to capture everything, and Abigail pointed out that this is not something you can optomize at compile-time without solving the halting problem.
So in Perl 5, I try and use built-in stuff even if in clever ways because it's faster than coding an explicit loop; in Perl 6 that will not be the case, but will I avoid grammars like the plauge because they are far slower than procedural code? That's my worry.