There's no reason that perl 6 patterns will be any slower than perl 5 regexes. The equivalent constructs should take about the same amount of time, modulo the odd optimization on either side. (Likely the perl 5 regex engine will be faster by a bit to start, if for no other reason than a dozen years worth of vicious optimization inflicted on it)
I'm not sure where you came up with the thought that you won't be able to limit what gets captured--you'll have at least as much control over capturing as you do now, if not moreso.