Yosemite is a total mess, but it has gotten slightly better with 10.10.3 (if you are willing to ignore the Photos app, which launches constantly for no reason).

If you are willing to mess with drivers, you may want to try installing Linux. On a 2013 Air, battery life and performance (even graphics!) are surprisingly better with Debian Jessie than with OS X. Of course, it is a newer model than yours, but the drivers will only be more stable and mature with a 2011. Especially if you have 4 GB RAM and you're running into a memory barrier with OS X, you should see a significant improvement.

My Photos app was launching because it got reset at the default action when my iPhone connected. It's possible that's what is happening to you too.

My iPhone and a couple flash drives do this, probably because it thinks my scanner is a camera. Unfortunately I can't stop it from launching (assuming I can disable it in Preferences) until it will actually launch—I have left it beachballing for a very long time and it still has not finished launching, so I just force quit it.

Open the Image Capture application, and you can change the setting at the bottom-left of the window. It seems to get increasingly hidden with each OS release.

This used to be the only place to change the setting—which was ridiculous because virtually no one ever used Image Capture (especially because iPhoto opened by default). But to Apple's credit, you can now also change this preference in Photos. Of course, as the previous poster pointed out, you have to actually wait for Photos to finish launching. But at least it's there now!

If you want to stay on OSX:

Always 'Quit' open apps you don't use, never, NEVER have 2 sessions open at the same time, and try using lighter applications when you can (VLC instead of iTunes for music, Skim instead of Preview for PDFs, Preview instead of Photos for images, etc)

I have to use a 2012 MBP with Yosemite from time to time and if I follow these rules it is definitely bearable.

I recommend VOX over VLC because it has Soundcloud integration and looks nice.

My 2011 11" Air (i7) runs 10.10 just fine. I haven't noticed any slowdown over the past 4 years, although I do disable all the unnecessary transparency and animations Apple keeps adding for seemingly no reason.

By the way, my battery health is also around 85% after 500 cycles and being plugged in and charged to 100% whenever possible.

Switch to OSX 10.9, problem solved. My 2012 rMBP with top end specs is also sluggish on 10.10.3. Not sure what Apple is thinking. On the other hand I am still using my 2003 Panasonic CF 73. It's not quick, but for the limited tasks of automating my DSLR it works perfect.

Define unbearable. Up until 10.10.2 the OS X window server was a bit sluggish. Now it's tolerable for my purposes.

MacBook Pro (13-inch, Early 2011) 2.3Ghz i5, self-upgraded hardware to 16GB 1333MHz DDR3 RAM, Samsung SSD 120GB, caddy with 500GB HD, replaced battery with cheap Chinese, running 10.10.3 like a charm (only my SD-card slot never worked).

Yes, mine still runs smooth, that's why i always buy the fastest model (processor) available, it's more expensive when buying but usually saves money in the long run. (i have the 1.8GHz Dual-Core Intel Core i7)

my macbook 2008 running os x 10.7.5, 128gb ssd and 4gb ram is still fast enough and gets the job done. It's even faster than the latest middle to low end windows PC's (dual boot with windows 7)

MBP 2009 here... works as good as new. Only upgrade was a SSD for the hdd. Still only 4GB RAM. Wouldn't want to do much video/graphics processing but everything else is fine.

Mid-2010 MBP upgraded with SSD runs perfectly on 10.10.3

Same here, only boots extremely slow (I have the original HDD but 8gb memory)

You should really upgrade to SSD. Makes a huge difference (easily 5-10X speed-up for many tasks).

I run Windows on mine and it runs fine (late 2010 13", 4GB model). Only use it when travelling though.