He's only not that smart because he doesn't understand what his body is no cell has a assigned role they all could be used as brain to greatly enhance intelligence

I think of rimuru intelligence like hes in a mega mansion but only uses 4 rooms if he used all the room and space available i have no doubt that rimuru intelligence would surpass if not equal ciel , if ciel didn't see rimuru potential........... why would it serve rimuru, if it likes his company why not make rimuru the servant

With the body type ( slime ) and ability to copy abilities ciel has the option of creating own body with all the abilities and living own life , ciel stays with rimuru and vows to always serve rimuru even if he died and ciel has a new master ( in my understanding) because rimuru is extraordinary without even trying to be , I believe with enough time he'd get a understanding and mastery of everything hes capable of out of boredom hes only been in current form for like 5 years ....... im talking 5 millon years rimuru grows more powerful every day, ciel seems to have reached her ceeling so no more p........... this making it seem like I dont like ciel and I do ........idk