Not only negligible, but non-existent. Even if mIRC chooses to run /debug, it's still parsing the conditional-- you could represent it by the pseudocode:
if (debugEnabled()) runDebug();
Even if debugEnabled() is slow (it's probably not even a function, but just a BOOL or int value), both code paths execute the function, so it's still: <delay> <runDebug> or <delay> <runEvents>.
The only case where it would actually
be slower would be if you had /debug -i on for something else, in which case mIRC would process as: <delay> <runDebug> <runEvents>. But since you're using /debug already, you probably don't want to replace it with event handling code anyway.