I think it's nice to have back in our browser the game, but I think you're getting unity wrong way.
You're not using their optimized stuff, but you're relying on your old tiles.
this way it's not optimizing anything and it's resulting very complex for the engine.
I think you should take more time, and try to re design stuffs using their optimized engine.
Don't worry about having it look like bit different, it will be better for sure, and mostly NEW
This is my opinion. Don't copy and paste old tiles, make new optimized ones.