Hello, first time poster here.
I just did a few tests with Final Fantasy XIV Stormblood Benchmark (for those who don't know, it has a loading time benchmark). Here are the results.
Samsung 970 Pro 512GB NVMe
Scene #1 1.633 sec
Scene #2 2.325 sec
Scene #3 1.737 sec
Scene #4 2.740 sec
Scene #5 4.652 sec
Scene #6 0.956 sec
Total Loading Time 14.046 sec
Samsung 850 EVO SATA 250 x2 Raid 0
Loading Times by Scene
Scene #1 1.891 sec
Scene #2 2.510 sec
Scene #3 1.860 sec
Scene #4 2.994 sec
Scene #5 5.375 sec
Scene #6 1.075 sec
Total Loading Time 15.707 sec
WD Black HDD 7200 RPM (drive is kinda old and 99% full so I think it's especially slow. Should probably be faster if it's a new, empty drive)
Loading Times by Scene
Scene #1 8.836 sec
Scene #2 7.554 sec
Scene #3 7.001 sec
Scene #4 9.182 sec
Scene #5 16.503 sec
Scene #6 3.348 sec
Total Loading Time 52.429 sec
And just in case someone wants to see results for a RAM Disk (SoftPerfect Ram Disk 3.4.8)
Scene #1 1.294 sec
Scene #2 1.908 sec
Scene #3 1.429 sec
Scene #4 2.230 sec
Scene #5 3.655 sec
Scene #6 0.786 sec
Total Loading Time 11.303 sec
So a decrease 15.7 to 14.0 seconds is still about 11% difference. In another test I was doing with the same FFXIV benchmark, it got about 14% difference. So I think there's still a difference from SATA SSD to NVMe. From those youtube comparison videos, I'd like to say it's more like 5-10% difference. Some people say you're not gonna feel the difference. Yeah, I guess that's true, but some people probably won't feel the difference between playing at 60fps and 54fps, but 10% is 10%. Probably not worth the premium just to get a NVMe just to shave off 2 seconds from a game, but if you're building a new computer, NVMe is probably nice to have.
But for the love of god, please don't play on a HDD.