As a PSA (public service announcement), I'm reporting here that updating your Yosemite system to 10.10.3 is incredibly toxic if you use WiFi.
I've seen other reports of this, and I've experienced it myself. What happened is that the update for 10.10.3 seems to have done something tragically bad to the WiFi drivers, such that it completely hammers the network to the point of making it unusable for everyone else on the network.
I have late 2013 iMac 27", and after I updated, I found that other systems started badly badly misbehaving. I blamed my ISP, and the router, because I was seeing ping times of tens of seconds!
(No, not milliseconds, seconds!!! In one case I saw responses over 64 seconds.) This was on other systems that were not upgraded. Needless to say, that basically left the network unusable.
(The behavior was cyclical -- I'd get a few tens of seconds where pings to 188.8.131.52 would be in the 20 msec range, and then it would start to jump up very quickly until maxing up around a minute or so. It would stay there for a minute or two, then rest or drop back to sane times. But only very briefly.)
This was most severe when using a 5GHz network. Switching down to 2.4GHz reduced some of the symptoms -- although still over 10 seconds to get traffic through and thoroughly unusable for a wide variety of applications.
There are reports that disabling Bluetooth may alleviate this, and also some people reported some success with clearing certain settings files. I've not tried either of these yet. Google around for the answer if you want to. For now, my iMac 27" is powered off, until I can take the chance to disrupt the network again to try these "fixes".
Apple, I'm seriously seriously disappointed here. I'm not sure at all how this testing got past you, but you need to fix this. Its absolutely criminal that applying a recommended update with security critical fixes in it should turn my computer into a DoS device for my local network. I'm shocked that several days later I've not seen a release update from Apple to fix this critical problem.
Anyway, my advice is, if possible, hold off for the update to 10.10.3. Its tragically, horribly toxic not just to the upgraded device, but probably to the entire network it sits on. I'm a little astounded that a bug in the code could hose an entire WiFi network as badly as this does -- I would have previously thought this impossible (and this was part of the reason why it took a while to diagnose this down to the computer -- I thought the ridiculous ping responses had to be a problem with my upstream provider!)
I'll post an update here if one becomes available.