If I understand correctly you have 1 host with 15 VM's. I assume they are sharing the same datastore.. There is your problem.
Your disks array or possibly you don't have enough spindles, can't keep up with the IOPS to sustain all of these VM's. You need to increase the number of spindles, or upgrade the disks \ controller. SATA is not the same as SAS, even NEAR line.. that's a marketing gimmick.. it's not "NEAR" anything... Only true SAS can give you good performance, but 1 or 2 disks isn't good enough, 8 minimal..anything more than that is much better.
That's where your problem is, you can move the VM elsewhere but that is not solving the problem with that host.