To answer your questions.
- Will the writing speed decrease to usb speed?
Yes, data will be distributed over all disks including the redundant part. There might be situations where only the two fast internal disks are involved, but in general a read or write would have to wait for the slowest disk to finish its operation.
- What happens if someone turn the disk off and on again during operations?
The mirror will be in degraded status when the disk is turned off. When it is turned on again, the mirror will very likely being resilvered.
- Was it a good idea overall?
My opinion is no, it wasn't a good idea.