I believe that is true, but it depends on the person, background, country and other circumstances. I know for some checks it requires a full FBI and INTERPOL check which can take weeks to months (case dependent of course) Eg; It is VERY difficult to background check people from many African countries due to the lack of systems in place to do so but someone from say, the UK could be very quick given the US and UK co-operate heavily.
But it is absolutely 99.9999% USCIS just being an incompetent agency if we are being honest! ha ha