Thunderbolt/USB is a mess on the M1 Macs. Reports everywhere of speed issues when connecting directly to a M1 Mac or through a TB4 or even TB3 dock. I can confirm this owning an OWC TB4 dock, a CalDigit TB3 dock and another TB3 dock.
Speeds for SSDs can drop by half or to a quarter of what they should be. The SansDisk SSDs and Samsung SDDs seem to react differently depending on the model and if they are attached directly to the M1 or a TB4 or TB3 dock. I’ve tested over a dozen cables and that’s clearly not the issue.