For a RAID1, most RAID controllers (and software RAID implementations) will absolutely read from all devices so as to service the read ASAP.
For distributed parity forms of RAID, you inherently have to read from all devices.
The problem is guaranteed with distributed parity raid; the controller will have to wait for the slowest disk to complete the read. Both reads and writes will be limited to mechanical disk performance levels.
With a RAID1 mirror set, you can get a performance improvement on reads since the SSD would presumably service all of them. Writes will still be delayed by the mechanical drive(s).
In addition, most RAID controllers do not support mixing drive types. Most of them don't even recommend mixing drive speeds (e.g., don't even mix 10K and 15K RPM drives). So you are proposing a whole new product essentially, and the expected gains are quite minimal.
Rather than investing in new tech with questionable benefits, implement an existing solution. You can choose to setup a hot sync, cold sync, or backup device for a standard SSD array that is well-tested and performant.