The proposal outlined above doesn't introduce runtime reflection, but rather aims to be an equivalent of static or compile-time reflection. While we don't include any direct static evaluation guarantees in the text, we believe this can be added in the future either through improved optimizations at SIL level and/or more explicit addition of guaranteed compiled-time evaluation to Swift language.
As outlined in my previous comment, Swift is already doing a great job at optimizing a simpler version of the encoding, so there is a some evidence that suggests that more work on optimiziations at SIL level could be sufficient to minimize (or even completely eliminate) the current performance gap.