RS2WC040 is an entry-level controller. It does not have/support batery backup-unit, and it has "no" cache. More exactly, it has some "context" cache of minimum size (2MB), but not "true" cache (half GB or more), as medium-class controllers. Even if you are using SSD, cache on controller is still needed for parity calculation. If there is no one, then raid5/write performance is low.
I'm affraid it is something you have to accept with this controller. You can increase write speed for the cost of some drive capacity, using raid10. Apart from that, you can not do a lot, as you can read on this link: