it depends where exactly you put the command, it is applicable for all actions below it (until you don't switch it off), so if you put it at the top, it will be true for all actions, but maybe it won't be able to help and the issue is related to imacros itself, not the macro
are you talking about the standalone imacros browser or the firefox addon? i'm just asking, because if it's the former, you should try the firefox addon instead, if you don't run tasks where the standalone version is required by all means, the firefox addon runs a lot smoother, the standalone version is based on IE, go figure...
i also experienced with the standalone version (both version 9 and 10), that if i run a macro in loop, after some loop the browser becomes unresponsive