Anytime I upload something, my internet slows down extremely. What can I do?

You ran a speed test whilst uploading. The upload then showed 0.3 mbit/s. I assime your file upload speed to the server was somewhere near (2 mbit/s - 0.3 mbit/s), right? As jjlin saus: downloading also needs uploading. Limit the upload speed to your server (sftp -l) for example to 1.5 mbit/s and then see if your downloads run smoothly again when you are uploading.

I run OpenBSD on my egress device at home and have had this addressed for quite a while. I recommend you read about this well-known phenomenon at https://web.archive.org/web/20120923235002/http://www.benzedrine.cx/ackpri.html for more information on how to address this using pf (the author is the original developer of pf).