I recently picked up an SSD and enclosure with a built in USB Cable. CrystalDiskMark shows a >500MB/s read speed and >400MB/s write speed with a direct USB C connection to a USB 3.1 gen 2 port, so all good here.
But I also picked up these adaptors from amazon.co.uk to ensure I could connect the drive to more common USB type A ports. These adapters noted that they could handle the 10Gbit speeds of USB 3.1 gen 2, although they do note that when this is the case the "other side" (Type A side) runs at USB 2 speeds. Out of a lack of options I decided to give them a go nonetheless and the claim is true. But it's not as simple as saying "these must really just be USB 2 adaptors with a blue finish".
When using the adapter to connect the drive to a front panel USB 3.1 gen 1 port connected to the USB 3 motherboard header, the speeds are ~300MB/s and 400MB/s read and write respectively. So the adapter can handle high throughput but just can't when used in gen 1 or 2 ports on the IO panel of the motherboard itself. Maybe this is a motherboard issue...
Edit: Just verified this across 2 Desktop systems:
Rear Motherboard IO (USB 3.1 gen 1 and gen 2) with adapter: ~40BM/s R/W
Rear Motherboard USB C with no adapter ~500MB/s R and ~400MB/s W (full speed)
Front panel IO connected to motherboard USB 3 header (gen 1) with adapter ~200MB/s R ~400MB/s W
What am I missing here? I have found only one adapter available that clearly states it has 10Gbit ports at either end but it's pricey so any alternative suggestions are welcome. Notably, it has a significant rectangle shape at the USB C side, maybe this is part of the explanation...? See below: