I'm using OneDrive app from AppStore in macOS Ventura. While downloads are fast as expected uploads are awfully slow. I checked the app's preferences for any upload bandwidth limit and nothing is configured. But upload is around 10KB/s only.
However, when using the browser to access my OneDrive or a SharePoint document library and doing an upload there, uploads are fast. The browser upload easily goes up to 2-4MB/s. So nearly 1000 times faster than uploads using the OneDrive app.
So it's neither my network connection nor my system itself. Only the OneDrive app transfers very slow. CPU load is very low during upload and no virus scanner is active.