Somewhat. In practice 5400 vs 7200 isn't a such a big deal as it used to be if you're transferring data via 1 GB/s pipe. But, bear in mind that it's not just the speed of the parity drive in question. It's any drive involved in the write, and the slowest will dictate the write speed.
If the motherboard has a hardware RAID controller, then the answer is yes. Most motherboards though while they say they support RAID in actuality are a software RAID in which case unRaid won't recognize it. But, even if it is a true hardware RAID, then I would argue that by doing a RAID 0 as a parity drive, then you are doubling the chances of the parity disk "set" failing, which IMHO isn't worth the increase in speed.
Back in the day when drive speeds were much slower, then the use of a cache drive for user shares was important. Nowadays even with modest hardware on a 1GB network you get decent enough speeds that it's not so important, and many users (including myself) can't even be bothered to utilize the cache drive for anything other than as an "application" drive (ie: docker appdata) and simply write everything directly to the array. If pure speed is an issue then yes cache all writes to the array and you will see a modest improvement on 1GB network. 10GB network you will see a substantial improvement.