Recently came across some strange behaviour on a couple of OSX macbooks. Both laptops could access some websites but not all and both Safari and Chrome were affected.
After a while of digging around I realised it was sites on port 80 that I had problem with (so SSL sites were not affected). The problem was present regardless of wifi or wired network I was on. The browser would request the page and eventually it would time out and some times the laptop would become unresponsive (especially using chrome which refreshes in the background).
The laptop were new and running ESET mac internet security. Having looked at the ESET logs I realised that the eset proxy process was crashing on requests on port 80 and getting restarted stopping my browser from working until eventually my laptop would become slower and slower or crash. The error logs showed ESET Daemon Child process proxy did not handle signal restarting as per screenshot below.
To make things more frustrating switching off web access protection from Eset antivirus UI didn't fix the problem and to fix it, I had to switch of web access protection
I also had to go to web access protection->setup and select a port not used by the browser (I setup port 79, make sure you select a port not used) as per below screenshot.
I waited a few minutes for the eset proxy processes to die and browsers started working fine and not had any issues with browser or slowness anywhere.