Multipart upload performance issues to Digital Ocean Spaces from browsers on Windows

Question

I've run into a really strange issue with uploads to Digital Ocean Spaces.

We'd been getting more and more complaints from users (starting beginning of November) that uploads were really slow. We couldn't reproduce any performance issues on our machines (Mac) but couldn't really ignore the complaints so I began to dig a little deeper.

What we've determined so far

• Performance is 10-20x slower if the following criteria are met:
  • On Windows (we've tested with Firefox, Chrome, Edge). MacOS is fine.
  • Multipart upload PUT (with presigned url, but single file PUT is fine)
  • AMS3 region (all buckets there are slow, but we tested FRA1 which is fine)
• A 15MB single file upload will complete in under 3 seconds. A 15MB part of a multipart will also complete in under 3 seconds on Mac. On Windows under the conditions above: 25-45 seconds or even longer.
• In browsers only, command line api is fine. (Main difference is presigned urls?)
• It started around the beginning of November, where we see an extreme drop in overall multipart upload speed from Windows.
• ISP doesn't matter (we tested several)

We use Uppy for multipart uploads with an API providing the signed PUT urls. To rule out any issues with the code we've even written a quick proof-of-concept without uppy to do the upload, and it was still slow.

Does anyone have any idea what could be happening here? Can anyone reproduce this problem on their AMS3 space on Windows?

I'm currenlty in contact with DO support of course, but they take ages to respond and have not been helpful so far. I do suspect it's something on their end but it's just so weird and any push in right direction might also help them solve this faster.