Relatively is a modifier that here means "compared to something else". When someone says "relatively X", they are saying it is not actually "X" but it seems so

Relatively is a modifier that here means "compared to something else". When someone says "relatively X", they are saying it is not actually "X" but it seems so in comparison. So they are saying that the combined particle isn't sluggish (because 3km/s is very much faster than a slug), but in comparison to light it _is_ sluggish. Hence, relatively sluggish.