"...I have a LSI MegaRAID SAS 9260-4i without BBU..."
I think missing BBU might be cause of your problem. I have much older 8888ELP, I have been using it without BBU and suffered from performance too. When I added BBU and enabled write-cache performace skyrocketed. I'd recommend to invest in decent BBU. You can find some cheap 2nd-hand one on ebay...
BTW, what disks are you using? I'd recommend to use enterprise-class (or "raid-edition") drives, i.e. drives which are especially designed to run in raid. They have time-limited error recovery, or can even postpone error recovery and sector-relocation for later time, when drive is not being used...