The Turret behavior now uses collision cells to avoid having to check lots of objects. This is good for games like RTSs, where you have loads of objects. In this case you just have one player object so it doesn't benefit so much.
The problem is the turret range is set to 10,000 - which seems excessive, it's the size of the entire layout. This is inefficient, since surely you don't want to bother dealing with turrets very far away from the player? It would be faster to simply ignore those turrets.
Collision cells mean that checking a 10,000 pixel radius involves checking a few hundred collision cells. Since you've told every turret to detect range over the entire layout, then created over a thousand turrets, it's now checking hundreds of thousands of collision cells. Before collision cells it would have only checked the player directly, so it is indeed doing a lot more work. So this is indeed a worst-case-scenario for collision cells. However if you had a reasonable turret range and hundreds of Player objects spread out over the layout, collision cells would make this far, far faster than not using collision cells (as is true of many other cases).
Basically this is mis-use of the turret behavior. Hit the range down to 1000, and the game appears to function identically but at an easy 60 FPS. If you could reduce the range further it would be even more efficient. If you want to detect the player over an infinite range, don't use the turret behavior - just use an 'every tick, rotate towards player' event (which effectively does a turret behavior over infinite range while circumventing collision cells).
Collision cells do make a trade-off, and some edge cases like this may be slower, but it should be easy to work around. Note this is also specific to the Turret behavior's use of collision cells, and does not affect collision cells in general. Closing as won't fix.