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Yes, this problem is understood and a bug is filed (15466549).
Writes will go to both sides of the mirror and will wait for acknowledgement. Reads will alternate.
If possible, I would put your most important data or data that needs the best performance, in a pool on the faster new devices.
This would mean keeping your slower old disks in one pool and new, faster disks in another pool.
Another idea is to use the SSDs as either separate cache or log devices, depending on whether you need faster read or faster write performance.