I have the same issue with a Samsung 960 EVO 512 GB and I am also looking for an explanation for this phenomenon. The processor is by far fast enough to enrypt decrypt without the drive having to take a performance hit. I went looking for M.2 NVME settings in the BIOS and System preferences and also for any setting in Veracrypt itself which might negatively affect the performance but so far no luck. Does anyone have any helpful input on this? Is it to be expected?