This is a very good question. It's a fairly subjective situation, though. Here is an excerpt from the Universal Tournament Rules:
Players must take their turns in a timely fashion regardless of the complexity of the play situation. Playing too slowly or stalling for time is not acceptable.
It's clear that slow play is not limited to simply delaying the inevitable. I think most judges will give some latititude if the situation is highly complex, but the rules _do_ state that complexity is not a mitigating factor in determining slow play.
Honestly, I think your opponents have a point. If you cannot reasonably expect to complete three games in 50 minutes (
knowing that such a time limit is in place), then you should either learn to play the deck faster or select another deck. It sounds harsh, but time limits are a necessary restriction in the game, and you have to anticipate that.
As for a chess clock, or something similar, I just don't think it's practical in real-life Magic. You just can't hit a timer every time you pass priority. You can't really do it based on turns, either, since your opponent can spend more time making decisions on your turn then on your own. Draw-Go, for example.