In something like d20 it works, most of the time. Personally I give everyone an amount of time to do their action. The only person who should be going "um..I'm...um..." is the first person in combat order. While everyone else is going you need to be looking at your #s, checking your powers/spells, etc. If you don't have an idea when I get to you it's a held action, next person. If you still have no idea, you've lost it.
Mean? Possibly - why should I allow players to waste mine and the other players time with it.
Me: "Ok Keith, you're first to act. Do you want a min to consider options? Or would you like to hold and come back?"
K: "I'll use my Pulsor blast against the first agent to the left" ( so we figure range mod, attack roll, dmg, knock back...a good 5-15min depending (??))
Me: Ok David, what are you doing?😧
D : "Um...wait..uh..let me check something...um, ok I'll do a move through and..." (10 min of him checking what a move thru does...)....
And yeah, nor everyone knows the rules, starting players, etc etc...I can be lenient. You add all those "um..uh (book check)..well I think I can still cast Fireball..no wait..." and it kills the flow as well as time.