I tried sand timers, but when a player finished early it was a pain waiting for sand to run out to start another player's turn (unless you have a 15 second timer, and then it's so fast it's not an issue).
Next, I tried a digital timer. Got a cheap one from RPGShop.com. Too fiddly (it was a poor design).
If you allow mobile devices at the table, these should all have sleek timer apps or functions.
However, the best solution for me is to use the time to plan my stuff and then, when I'm done, I'll give the player a five second countdown with my fingers and voice.
I use the time players spend thinking, looking up rules or hand-wringing to plan my foes' next moves, roll stuff in advance, and catch up on notes and other admin stuff.
I also have a little checklist I scan if I have extra time. "Has anyone/thing nearby noticed what's going on? What can you add this round to spice it up? R U forgetting a foe ability?" Etc.
I only spend a minute or less doing this stuff, but it keeps me on top of things as the rounds click by. If a player finishes fast, I stop whatever I'm doing and resume combat.
Players also use this time to ask me questions about stuff coming up on their turn.
It works well for my group. Maybe yours too.
Hmmm, I've wandered off point. You wanted to pressure players and get combat rolling fast. Best for that is a timer (I up-voted foxxtrot's suggestion) but have the players police each other. Competitive players will rule to the second, adding pressure, and it frees you up to do other stuff while you wait.