very many have noticed that the new network-manager appears to be quite a lot slower to connect to… just about anything, really.
the reason for this (as confirmed on #gnome-hackers after being suggested by ray) is ipv6 support: when network-manager connects to a network, it has to assume that there might be an ipv6 router advertisement. it waits for this (and usually doesn’t hear it). that’s the delay.
if you don’t care about ipv6, the fix is quite simple: in network settings, under wireless, click “options”, go over the ipv6 tab and select “ignore” from the dropdown.
that fixes it up for one connection. i’m not sure if there is a way to do it globally.