Setting up hardware Raid 5 cache question

I recently purchased a 3 ware 9590se 16ml card on ebay and I build a raid 5 array with 4 2tb hard drives but was getting super slow transfer speeds as read and write, around 3-5mb which was weird to begin with but the end of the world for me I'm more so worried about loosing data. Doing some reseach I found adding a cache ssd will greatly improve speed. I want to buy a 60ish gb ssd to add as a cache. My question is if I try and transfer a file that is larger then the cache drive am I still going to run into that slow 3-5 speed or will the ssd help no matter the size of the file.