We haven't had any slow issues. Our virtual servers are hosted on fast 4 lane 6GB SAS Direct Attached Storage SAN, actually our entire SAN is built on direct attached using SAS Switch's. The other replicated servers are miles away from us but we have a dedicated 30mb/s fiber link between the 2 sites. It could be Amazon and the way their storage backend is and the round-trip time between your hosts. I have not had any experience with Amazon or any other virtual server hosting. Most hosting companies limit your IO's too.
Have read through the Gluster forums? Poked around and asked questions there? What version of Gluster are you using? Moodle version? We are still at 2.2.10, I need to setup the latest moodle on gluster to see how it performs with the new MUC since I have seen many reporting it has been much slower than 2.2 as it is.
Amazon has there own Gluster storage, since you are on Amazon you can get support through it if you are using their engine,
https://aws.amazon.com/solution-providers/isv/gluster
Actually supported through RedHat also,