I recently put together a new build with an ASUS TUF X570, 3700X and GeForce 2070 SUPER. I rounded it off with a 970 EVO SSD.
After installing the OS and drivers, I ran through a few benchmarks and I'm only getting 1600 MB/s, when I see reviews and others getting 3500 MB/s.
What could be wrong?
First, congrats on the new build! Researching your motherboard, it does appear you have two m.2 slots, one around the middle of the board that takes lanes from the CPU and one at the bottom that pulls from the X570 chipset. I would start by making sure you are using the top slot for your 970 EVO.
Next, I would recommend you put Windows 10 power mode into high performance and venture over to device manager and make sure write caching is enabled and the second box for cache flushing is ticked as well. You can find these options by right clicking on your SSD and choosing properties.
If neither of the above options help with the performance loss, I'd look and make sure you have the latest UEFI update for your motherboard, as ASUS does update regularly.