Why does "fio write" change the performance of "fio read"

I have a P4600 2TB NVMe SSD. Right after I used Intel SSD toolbox to secure erase it, fio reports 4kB random read at 809K IOPS.

sudo fio --name=dummy --size=10G --filename=/dev/nvme0n1 --ioengine=libaio --direct=1 --rw=randread -bs=4k** --iodepth=64 --numjobs=4 --group_reporting

However, after I tried to pre-conditioning the SSD with the following write command

sudo fio --name=dummy --size=2000G --filename=/dev/nvme0n1 --ioengine=libaio --direct=1 --rw=write -bs=4k** --iodepth=64 --numjobs=1 --group_reporting  (I also tried --size=1700G, the result is the same)

the randread command above is now reporting 4kB at 520k IOPS.

So why such a severe drop?