Why is uploading to S3 so slow?

Just because you can go up to 50mbps does not mean you will always get 50mbps; The network path to S3 as well as the latency are also important.

If you are able to use multi-part upload, you will be able to break a file down into multiple pieces and upload it using multiple threads, possibly increasing the upload speed.

Just in case anyone stumbles upon this....

I had an issue where it was really slow to upload from an ec2 instance to an s3 bucket, turned out to be really simple, the region of the bucket! I was using ec2 instances in north california, when the bucket was created as us standard it was really slow to transfer, with the bucket set to north california it was heaps faster.

I have seen many threads in a forum about slow upload to S3 using various clients, such as the free command line tools (written in Python, btw) and commercial ones.

Although I have no hard evidence for you, switching the S3 client (e.g. Cloud Berry Explorer) might solve your problem. Try it! :)