Combinator attack slow down

Hi! I'm using the combinator attack with cudahashcat. I have a gtx 680. For MD5 bruteforce I get 2700 MH/s. If I try combinator attack with 2 dicts (20k words and 120M words) I get 1500MH/s (I believe it is because i'm not using any rule?)
The question is why towards the end it slows down to 1000 KH/s?



Probably I'm doing something wrong, I've got alot to learn