The number on the box is your SSD's cache speed. This test writes continuously — gigabyte after gigabyte — and charts the speed curve until the fast cache runs out and the drive shows its real sustained pace. The temporary file is deleted automatically when the run ends.
A healthy modern NVMe drive holds a high plateau for tens of gigabytes (the SLC cache), then steps down to its native flash speed. Where that step lands is the number that matters when you copy a game library or record long footage: a drive advertising 5,000 MB/s might sustain 1,500 — or 350. Drives that are nearly full show smaller caches and earlier cliffs, and some budget drives dip further once the cache must be emptied while writing.
One run writes the amount you choose once, then deletes it — roughly what installing one big game writes, against drives rated for hundreds of terabytes. The browser writes through a real file (flushed regularly so RAM caching doesn't paint a fantasy curve), but the OS still sits between the page and the hardware: treat the shape of the curve as the finding, and the absolute numbers as close-but-conservative.
Most SSDs write into fast SLC cache first; when a long transfer exhausts it, speed drops to the native flash rate — sometimes from thousands of MB/s to a few hundred, mid-copy.
One run writes your chosen amount once, then deletes it — about 0.01% of a typical drive's rated endurance, similar to installing one large game.
Early writes can land in the OS's RAM cache. The test flushes regularly, and the longer it runs the more the curve reflects the drive itself.