Gail and Jeff hit the nail on the head in the first two replies.

With the benefit of all the posts ahead of mine, I suspect your actual question is closer to "My IOStall for writes is much slower than for reads; what can I do to improve performance".

I can say that RAID5 parity with a modern controller is extremely unlikely to be the cause of the slowness; evidence from a TempDB (one data file, one log file, both on the same RAID5, with 6 dedicated SSD's) is that even with signficant usage, average IOStall for reads is 2ms, and average IOStall for writes is also 2ms (2TB of total reads, 3TB of total writes since last restart).

I'd definitely look into whether your spindles are dedicated or not - if they're not, your measurements are necessary but insufficient. I'd also check into all the usual suspects first - wait types, index fragmentation/split IOs, filesystem (NTFS) fragmentation, etc.