I know that in another text adventure builder I use, the timers can be rather unpredictable for others when they play your game. I suspect this is down to a number of things - the weight of the game, the users' internet connection, etc.
Because my game employs the <<timed>> macro fairly often - often in situations where it's important the action is triggered after the specified time - I thought I'd just check if Twine is known to suffer similar issues.
They work correctly for me, but I suspect that's because my game is constantly cached.
Comments
The story HTML file you create is a web-application, and as such it has all the multi-threading limitations any web-application has. A timer event generally happens on (or as soon after as it is feasible) the time period indicated, this assumes that the GUI is not busy doing other things.
As far as caching goes. Your project is cached, in the sense you mean, for everyone. The only things that get loaded separately are any external resources you've specified. The story format itself and your project's passages, including the code within them, cannot be anything other than fully loaded.