V4 registration should be neither significantly slower nor take significantly more memory, if the settings are equivalent.
SetMetricSamplingPercentage should have an effect. See a related discussion.
The old registration classes are still supported, so you don’t have to switch to V4 registration framework. Unless you have a good reason to switch to V4 framework, I suggest you stick with the old one. V4 framework is more flexible (and complicated), with many differences including different defaults for some things. If you switch to V4 registration framework, you will have to re-tune your parameters, at least to some degree.