I agree with this only if make it exclusive to the Moon Kick and not his overall speed.
I don't feel like it's a good idea to scale Boros's Regular speed to the Moon kick he performed
only once, which is also more of a strength feat than a speed feat.
And it's visibly his best striking/speed feat in the manga that was performed only once, nothing of this magnitude was ever displayed by him later.
1.
It is the only kick that created an energy orb around both Boros and Saitama indicating that kick released more energy than his regular punches.
If we can use anime as supporting evidence, the Moon Kick was charged, created the same energy field around both Saitama and Boros and released massive lightning.
This further supports the fact that it's much more impressive than his other kicks and punches, meaning he can't move his legs or hands at that speed all the time, at least we don't have feats or statements that support it.
2.
Boros was visibly out of breath, meaning the kick exhausted him. He stood and started fighting Saitama after this but
non of his attacks even phased Saitama or created energy fields and lightning.