Ridiculously slow upload speeds both when using the desktop app, and website.

I had the same inssue and after a lot of attemps, contact with support, and a lot of seach, I found a soluction by a user here.
Try this:

1. Open CMD as ADM
2. Type: netsh interface tcp dump

3. Then: netsh int tcp set global autotuninglevel=normal

4. Restart the computer

Let me know if it helps!

There are several possible reasons why Dropbox upload is very slow, such as:

Your internet connection speed or bandwidth is limited by your internet service provider (ISP) or other factors.

Your Dropbox desktop app preferences are set to limit the upload rate or use the automatic option, which may not optimize the upload speed for your connection.

Your files are large, complex, or have many changes, which may take longer to sync or upload.

Your computer or device is running other applications or processes that consume a lot of resources or network bandwidth, such as streaming videos, gaming, downloading, or antivirus scanning.

To make Dropbox upload faster, you can try the following suggestions:

Check your internet connection speed and bandwidth using a speed test tool such as Speedtest. If your speed is lower than expected, contact your ISP or troubleshoot your network issues.

Adjust your Dropbox desktop app preferences to increase the upload rate. You can do this by following the steps in this article. You can choose to not limit the upload rate or set a custom limit that suits your connection speed. However, note that increasing the upload rate may affect the performance of other applications on your computer or device.

Reduce the size or complexity of your files before uploading them to Dropbox. You can use compression tools such as WinZip or [7-Zip] to create smaller zip files that can be uploaded faster. You can also split large files into smaller chunks using tools such as [HJSplit] or [File Splitter].

Pause or close other applications or processes that use a lot of resources or network bandwidth on your computer or device. You can use the task manager on Windows or the activity monitor on Mac to identify and terminate these applications or processes. You can also pause syncing temporarily on the Dropbox desktop app by clicking the Dropbox icon in the taskbar or menu bar and selecting Pause syncing.

Use a third-party tool to upload files to Dropbox faster. There are some tools that claim to speed up the upload process by using multiple connections, parallel uploads, resuming interrupted uploads, and other techniques. Some examples of these tools are [Cyberduck], [Zoom Uploader], and [MultCloud]. However, note that these tools are not endorsed by Dropbox and may have some limitations or risks.

That's already set to normal on my computer, that's the standard setting for Windows 10 I believe. 

It's nice that you all are trying to help, but I have a feeling that you are not reading what I initially wrote: My upload speed is fine, uploads of the same file onto other systems work within minutes. It is only Dropbox where the upload does not seem to work.

I also am not uploading through an app but natively through a browser.

Thanks for the update, lady8jane.

Do you see the same when using a different connection/WiFi?

And does the issue exist only on this specific device or other ones too?

It just suddenly started working again with no changes on my end.

Happy to see that, lady8jane!

If you come across this issue again, please let us know.

Have a great day!