With Starship it appeared as if it was struggling to lift off, staying for a while burning fuel until it left the launch pad and even then it felt a bit slow at accelerating.
I'll see tomorrow if it was just an illusion.
For Falcon 9 - it will look slower on takeoff as well. One reason not mentioned yet is that after engine start, Falcon is held down until all vehicle systems are verified as functioning normally before release for liftoff. So they can do pressure / thrust etc checks on the ground before releasing.
With solid rocket motors especially - once you light those you are pretty committed.
Starliner is also held for confirmation of all systems being functional even with the SRBs. Better to let them burn out on the pad and clean up the mess than to risk release with, say, asymmetrical thrust. That said I'm sure there are variances between the two rockets' procedures which could make one appear faster.
I think SLS does 90% thrust on main engines - then lights SRB's but SRB ignition is takeoff. A problem at that point is launch escape abort (capsule to safety) if for example one SRB lit but the other didn't for some reason. Not sure it can otherwise manage the asymmetric thrust.
Although you are a bit off the mark, you did actually hit on the likely reason for the faster liftoff, which is that the SRBs have a much higher thrust-to-weight ratio than the SpaceX Superheavy booster.