Promise Pegasus R4 slow read speed

My Pegasus R4 is set up as a RAID 5 and I'm getting fast write speeds... they seem to fluctuate a bit, but averages around 475-500 MB/s.

However, my read speeds seem to be around 200 MB/s. On the Black Magic Disk Speed Test, I get a LOT of X's for Read speed for various video formats. I'm using this for video editing, so this isn't good.

Right now I actually have two 2TB drives in the R4, along with two of the 1TB drives that came with the unit. All drives are SATAIII 6Gb/s and 7200RPM. Could these slower speeds be because I have mixed capacities? It appears that the RAID is only using 1TB of the 2TB drives, anyway, so I don't feel like this should be the problem. Nevertheless, I have two more 2TB drives arriving (same brand and type as my current ones) and I'll see if that helps.

Otherwise, does anyone have any idea what could be causing it or how to fix it?