RAID is generally slower at writes, since it writes the same information across multiple devices, and for some RAID flavors calculates and writes a checksum as well. Of the RAID flavors that provide true redundancy, RAID1 is generally faster at reads than a single disk since the information can be pulled from both copies at the same times.
RAID0 ("striping") is also
faster than a single disk, but it provides no redundancy. It's not a solution I would choose since I'm generally more concerned with reliability than performance.
I'd be surprised if the disk performance by the guest is notably degraded. Sure the data has to travel through the interface with the host, but that all happens in the CPU and memory. Since disk performance is largely a function of the physical device itself, I would think the guest probably has about the same read/write speeds as the host.
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