Why is my Project Rene save progress stuck at 99% for minutes?

Saving a massive community was maddening; the progress bar would just sit at 99% for ten minutes straight. I tried running a disk defragmentation in Disk Management, which I later realized is totally useless for NVMe and just added unnecessary wear. I used GamePP NVMe Queue Depth Configuration to increase the queue depth to 2048 and switched to high performance power mode. It finally worked. DiskSpd showed random writes hitting 52-60 MB/s, CrystalDiskMark tracked the 1500MB/s to 120MB/s drop, and HWMonitor saw 40-46℃.