Why USB File Transfers Start Fast, Then Slow to a Crawl

The burst you see first is not the device

Windows does not write to removable drives byte by byte as you watch; it fills a RAM write cache first, and that cache is what produces the opening burst. Once it drains, throughput drops to whatever the flash inside the stick can actually sustain. Cheap sticks make this worse with SLC emulation: a small fast zone sitting on slow TLC or QLC memory, engineered precisely so the drive looks good in a thirty-second benchmark. When that zone fills mid-copy, speed falls off a cliff — often from 150 MB/s to under 10.

Portable SSDs run the same play with bigger numbers, then add a third act: heat. An NVMe chip in a fanless aluminum case starts throttling after a few minutes of sustained writing, and the transfer rate sags with it. So a mid-copy collapse on good hardware is usually thermal, while on cheap sticks it is usually cache exhaustion — same symptom, different physics.

The other suspects

Before blaming the stick, check the link itself. A USB 3 port negotiated down to 2.0 — through a hub, a front-panel header or the wrong cable — caps everything at roughly 30–35 MB/s real world, and Windows warns about it exactly once, quietly, in a notification you probably dismissed. Then there is the file mix: ten thousand small files cost more in metadata and filesystem overhead than one file of the same total size, and the FAT32 or exFAT tables on removable media make that tax steeper. Antivirus doubles the pain by scanning every file on arrival at the removable drive.

• USB 2.0 link — a 3.0 stick in a 2.0 port, an unpowered hub or a charging-grade cable caps you at ~30 MB/s
• Small-file armies — metadata, directory entries and allocation tables outweigh the payload when there are tens of thousands of files
• Antivirus on arrival — real-time scanning runs on every file written to the removable drive
• Cable and port quality — some USB-C cables only carry USB 2.0 data; use the cable the drive shipped with
• Competing traffic — other devices on the same hub or controller steal bus time mid-transfer
• A nearly full stick — flash needs free blocks to write efficiently; a 95% full drive writes far slower

What actually helps

Measure before you fix: the Performance tab in Task Manager shows the link speed and queue depth while a copy runs, which alone separates a 2.0 negotiation from cache falloff. For many small files, zip them into one archive first — a single 4 GB file routinely finishes hours before the same payload as 60,000 files. Plug portable SSDs into a rear motherboard port, with the original cable, and give a throttling drive a pause to cool between big batches.

Some things you cannot fix, only know. A ten-dollar stick will always collapse after its cache fills; that is a product decision, not a defect. And if a drive that used to be fast is now slow through the whole copy, on every port, the flash itself is worn out — no setting will bring it back.

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