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.
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