I'm experiencing the same issue, trying to stream to YouTube. My speed tests appear fine, when I check on my phone, but my upload speeds are horrible, on my PS4. Everything seems fine, until I attempt to stream, and then I get the "low bandwidth" error, from the PS4. This just started 2 days ago.
I also hate to jump to it, but I can't help to feel like this is comcast throttling me. It also doesn't help my impressions, now that net neutrality is dead now either.
I have run into this situation myself, and feel that throttling is actually occurring. As the net neutrality rules are repealed, this is a standard and legal practice, and one that is OBVIOUSLY being used by Comcast/Xfinity.
Here's something I have tried that works well: Use an a VPN service to obfuscate your your connection. If Xfinity's QoS systems can't determine the type of data you are download (e.g. streaming video), or if the VPN obfuscates the destination/source of your connect, they may not throttle it.
I did a test, and found that I got HORRIBLE connections to some sites that stream video (sites not partnered with Xfinity). But, if I ran that same connection through a VPN tunnel (at the same time, so variation in bandwith demand is not a factor), I found the connection to be a LOT faster and more stable. Logically, it should be slower through a VPN, because of the encryption overhead. But, it actually sped up my connection and made it much more stable. The only reason for this, I presume, is that Xfinity's QoS system didn't throttle the connection.