I don’t agree coffeeking. The seals on these are flexible rubber. They have plenty of tolerance.
You’re right, their sealing does not become any less effective at lower speeds, its just that the sealing is inadequate for most mtber expectations because that tiny rubber flexible rubber membrane has to resist substantial external water pressure trying to get past it, not just from over zealous pressure washing, but from crashing through puddles, river crossings, etc, all at speed.
These bearings have been co-opted from industrial uses. The use at low or high speed rotation is irrelevant. Irrespective of their seals, they’re not designed for specific use in an mtb application and then we’re surprised that that sometimes don’t last very long.