A time limit of 15-30 seconds per round for decision making used occasionally is annoying. Consistently imposing a time limit will be exacerbating.
I've imposed time limits on players' turns in combat once in a campaign where combat had become bogged down with inaction. I created a spell that "slowed thinking" forcing the characters to make decisions using less than their normal mental faculties. The characters were allowed a will save to avoid the effects of the spell. Players of affected characters got 2 seconds for each point of intelligence to decide what their character would do; failure to decide within the time limit resulted in the character taking no action. Only time spent deciding what their character would do counted towards the limit; rolling and resolving the effects of attacks and spells wasn't counted.
A spell block for this spell in D&D 3.5 would look something like the following.
Enchantment (Compulsion) [Mind-Affecting]
Level: current campaign spell level, class of NPC that has it.
Components: V, S, M/DF
Casting time: 1 standard action
Range: Medium (100 ft. + 10 ft./level)
Targets: All creatures in a 20-ft. radius burst
Duration: 1 round/level
Saving Throw: Will negates
Spell Resistance: Yes
Affected creatures have more difficulty making decisions than usual. They can take no action that an ordinary person couldn't conceive and describe in 2 seconds per point of the creature's intelligence score. Creatures whose actions aren't completely described within this time limit take only the actions described within the time limit.
I gave this spell to an enemy and hit the party with it at the start of combat. I wrote down their intelligences next to the initiative list and timed the decision-making part of each turn. The character played by the worst offending player made her save against the spell, but her animal companion didn't. The spell sped up combat for that encounter and was mildly annoying to players. For players having trouble making decisions, this spell was a huge daze effect, taking away one or two rounds of actions. Subsequent encounters suffered from less drawn-out decision making.