Any software based raid with parity will perform badly on writes because of the parity calculation, including Intel RST. For raid with parity, you need a dedicated RAID card in order to get decent performance.
Indeed, software based raid performs badly and should be omitted. However, many motherboards have on-board hardware raid controllers, configurable from the BIOS, not drawing performance from the CPU, supporting raid-5 with parity.
False, the world runs on software RAID. NetApp, Dell/EMC/EqualLogic, HP, IBM, Oracle/Sun - every enterprise storage vendor uses software RAID extensively, and exclusively once you get into larger systems (for which they charge millions of dollars). Hardware RAID chips are still around, but they're not used heavily anymore. Things like thin provisioning, data deduplication & data tiering require software, not some Adaptec or LSI chip. Hardware RAID is only used when software RAID can't - like boot volumes. Yes, a lot of the 'RAID' in BIOS chips is fake software RAID. Look at things like ZFS and mdam - software RAID that requires direct access to the disks, not an abstracted hardware controller. Every cloud provider runs on software RAID - AWS, Azure, Google, etc... YouTube learned early on that disk I/O is much better when it knows there's 10 disks to talk to, rather than the 1 disk that a RAID controller would present.
Microsoft had to architect the Storage Pools to work in a way that's safe enough for consumers who don't do things like have UPS'es & are prone to accidently unplugging something.
I would be interested to see if adding 1/2 NVMe drives to the pool would improve the write performance, but there are limits built in to handle consumer level issues.