I have my core MSFS files loaded on my C: drive which is a 1TB NVMe.
Initally, I had the content folder (Community and Official) on a 2TB SATA SSD. I eventually added a second 1TB NVMe and moved the content there. I immediately noticed load time improvement.
This graph shows you a decent comparison between HDD, SATA SSD and NVMe SSD. Real world performance will vary based on a ton of external factors, but this is pretty much what you can expect in terms of read / write performance. .
Keep in mind that an SSD (even NVMe) won’t make your sim run faster. That’s all CPU and GPU. But it will make things load much faster.
In this day and age, the only thing mechanical hard drives are really good for is cheap, high-capacity archival storage. That’s where you put backups, files you don’t access often, etc… And even that role is slowly being replaced with SATA SSDs as they increase in size and drop in price.