I've just experienced a couple of days of incredibly slow browsing (using Chrome mostly, but changing browser didn't help) on my home wi-fi. After trying many things like rebooting the router, removing devices from the network, diagnosing with iStumbler, requesting new DHCP leases, setting manual IPs, I eventually rebooted the machine and everything is back to normal.
What could cause this behaviour?
It's a mid 2014 Macbook Pro with Retina. The wi-fi is an ageing Dynalink that only supports 802.11b/g. I have a VPN installed but not active, and many apps:
It had probably been a few weeks or even a month since the last reboot.
Throughout all these problems, a Macbook Air on the same network worked just fine - so it's not a question of some weird network saturation.