@MAGICDAVESHROOM
I can see your point regarding graphics overloads possibly causing the turbo mode to terminate, but I only seem to get the slowdowns at points in the matches that involve the appearance of blastonauts.
It can happen to me at any point. Sometimes it's fairly early on in the match when there are fewer zombies on screen. Sometimes it's later on when there are hordes of them. Sometimes it doesn't even happen at all. Other times it can happen on multiple occasions through one match. Basically it's totally random and I only notice it's happened because the game suddenly seems to be going much slower.
As a point of interest, and something that MAY well give a clue as to why it's been happening, I've just realized that I get the same thing while farming for seed packets on Far Future world. I often use map 27. It's a 'Plan your defence' map where 5 Bug Bot Imps drop down on column one at several points during the level. To make it play faster I always do it on Turbo, but I've now noticed that as soon as those Imps appear the turbo mode switches off.
I now can't make up my mind whether it's the appearance of the Imps themselves causing it, or if it's down to a message appearing on screen that says something like 'A wave of zombies approaching' that's the culprit.
Maybe you could try playing that map to see if you get the same thing? If you do try it, make sure to fill column one with Bonk Choys, or Wasabi Whips if you have it.
Then keep your eye on the Turbo button to see if it illuminates each time the Imps drop down.