Download speed drops to near 0 when maxing out upload

If I max out my upload speed (~6Mbps, e.g. upload a video to facebook) my download speed drops to near 0, < 1MBps. This only happens when upload if maxed out. If I configure e.g. amazon photo upload to limit upload bandwidth usage so the upload is not maxed out, I have no issues and still get the ~200Mbps download speed.

This happens regardless of the devices in use, regardless of wifi vs wired, even after rebooting modem and router.