This is generally true; shared _disk_ will suck once you have multiple VMs accessing it because they'll each try to optimize for seeks internally, but the combined I/O stream will be relatively seek-heavy and wreck performance. Where I work we call this the "I/O blender."
Solid state storage can solve this, but then you have to balance cost, high-availability (if your data is on a direct-attached PCIe card, what do you do when that motherboard dies?) and performance.
This is the problem that a new generation of storage companies are trying to solve. Shameless plug: Pure Storage. We make a solid state storage appliance that uses compression and deduplication to bring down the price of flash while still beating the pants off performance disk arrays.
Plus, they have something that appears to make up for what they lost-- the hardware scout. It executes ahead in the instruction stream, preloading necessary data and instructions into the cache. There's lots of articles accessible via google if you want to learn more: Sun's Marc Tremblay has been giving a lot of talks about how it works.