I've noticed that, sometimes, a few fedora websites, start.fedoraproject.org for example, take a long time to load (more than a minute), and even end up with missing resources, like css or images, or doesn't load at all (this one is quite rare honestly).

I've made a few tests on my side with curl, and it seems like some proxies randomly drop connexion.
This is not specific to one proxy, and it doesn't happen all the time, so it's hard to tell where the real issue is.
But it seems like proxy04, 06 and 14 are most often impacted than the others.
I've also checked the nic counters on those, and there is indeed a high (and rising) number of tcp error reported (TcpInErrs), so maybe there is some resource starvation somewhere.