Any of the more CPU intensive stuff enabled on the USG, ie BWM, App patrol, Anti Virus etc. If yes, try to disable and see if it works.
I do not remember if you can turn of DoS protection on the USG like on the older ZyWALLs, but if you can look at that to.
"Perl is executable line noise, Python is executable pseudo-code."
No, there are no "extra" features enabled. Debug log shows all the time something about bandwidth, even if it is not enabled. Is that normal?