Almost every designer and acoustics specialist I've talked to pretty much agree that anything under 1.5 cycles is inaudible. Beyond that, for a ported Sub to track so closely and barely break over 1 cycle is pretty amazing.
FWIW, I don't own one and likely won't ever, but GD is not something I would use as an argument against this Sub.
Complaining about GD like this and arguing for better comes across like somebody that bit hard on REL's marketing. Comments about "fast" and "slow" Bass are pretty often disregarded here as marketing BS.
Certainly there was a time when Subs didn't perform well, but those times have changed pretty significantly. Unless a designer completely sh!ts the bed, anything modeled today and competently built will far exceed any such concerns. Welcome to 2024.