If I read this correctly, you have disk groups with 2 included disks in each, in RAID1, this means all your writes are going to a *single* disk only. The SSDs are used for read cache, so they can*t accelerate writes. Only the controller cache can (a bit). Once the cache is full, you directly write to a single physical disk.
Increasing the numder of disks (at least test with all your 6 disks in RAID10) or even tiering with the SSD will increase the performance, I'm sure.