It seems the web process is not actually hung, but it's doing a lot of work in an infinite loop or something similar. I can confirm this doesn't before r197549, tested on GTK+ port only. I've created a test pad here: https://etherpad.igalia.com/2091
I cannot reproduce with r198471 nightly on Mac.
Works here as well with current trunk. I'll try to figure out which commit fixed it, thanks for trying it out.
<rdar://problem/25266845>