Gentle person,
sadly this is a known bug for iPadOS 13 and, as it looks at the moment, iPadOS 14 (if you search for slow USB-C speed, file/media corruption you will get quite a lot hits on e.g. on reddit or Apple’s mailing lists)
For the moment none of the suggested solutions (different file system or cluster size on the external media) seems to work consistently. There is anecdotal evidence that in some circumstances it works - but in no reproducible way.
The only reliable way to copy a lot of files without corruption seems to be via WLAN (or LAN, which is so much faster, but both require additional hardware like a RAVPower WLAN-hub or LAN-adapter).
Everyone can and should report it here ->
https://www.apple.com/feedback/ipad.html
Or, if you participate on the beta or developer program, here ->
https://feedbackassistant.apple.com/
There is a quite old Apple Mailinglist discussion on this (slow USB-C transfer times), seems like not enough people reported this to Apple. Maybe we all should write to Hair Force One a.k.a. Craig...