Really? My experience with EBS has been more to the opposite... especially with write performance.
If you got to write data, you got to write data. Flakey performance becomes more likely as more volumes are added.
Previously I think AWS engineers recommended we use RAID0 with EBS... ugh. Don't care cloud or not, RAID0 on production DB servers sounds downright suicidal.
I don't know if that's what actually happening, but testing seems to confirm that RAID performance is more consistent than single drive performance.
RAID0 on EC2 isn't as insane as it appears at first glance. EBS's have an annual failure rate of around 0.2%, while hard drives are around 5%. So, the chances of 1 EBS drive failing is around the same as the chance of 2 physical hard drives failing (5%^2 = 0.25%). It doesn't sound unreasonable to suggest that a RAID0 of EBSs is about as reliable as a RAID10 of physical drives and more reliable than a RAID5 of physical drives.
While that's not the fastest thing in the world (e.g., a good SSD will outperform a 16 drive EBS Raid for most workloads), I don't think it's fair to characterize it as 'slow as molasses'.