NVMe SSDs have incredible burst speeds (7,000 MB/s), but after writing 20-30GB, the controller heats up and the SLC cache fills. The drive drops to "TLC direct write" speeds (1,500 MB/s).
Test: Use dd to write the 50GB file to the raw disk, bypassing OS cache. 50 gb test file
dd if=50GB_test.file of=/dev/nvme0n1 bs=1M conv=fsync
Watch the speed graph. If it collapses after 25GB, your drive needs a heat sink. NVMe SSDs have incredible burst speeds (7,000 MB/s),
Get-FileHash D:\50GB_test.file -Algorithm SHA256 Watch the speed graph
Pro tip: For frequently repeated tests, use xxhash (faster than SHA256).
Creating such a file can be done in several ways, depending on the desired outcome (e.g., random data, constant data). Here are a few methods: