MacBook – Wi-Fi drops since 10.10.3

If you have checked both the user keychain and the system keychain for old passwords, we're left with obscure guesses.

Two things come immediately to mind (from the obscure side).

1. Your mac will join any base station with that SSID. I have seen people literally setting up a second router in busy coffee shops - you have to hold the option key and make note of the mac address you have connected to and see it change to catch this.
2. If your wifi connects to the router wifi, it doesn't mean the dhcp server has any more addresses to lease. You will need the owner to reset or power cycle the router to see if the leases are all out and unexpired. You basicall have a great pipe connecting you to the main, but no water is flowing to you until you get a lease.

I'm experiencing some slowdown after a while even on my MBA. It's exactly how you described it. It's perfectly smooth after booting the machine but eventually, after using it for one day or so the animations (especially launching Mission Control, switching between spaces and full screen apps) get noticeably choppy. I don't think there's anything that can be done about it right now, I made a clean install and even executed an Onyx automation, but the problem persists. This is clearly an optimization problem in Yosemite and I really hope it will get better with the future updates.