Should I upgrade my MacBook Pro to the latest OS?

I recently upgraded to Mavericks with a newer macbook pro (2013 model), and it was awful. It had been fine pre-upgrade, and then suddenly everything was very unresponsive. https://ask.metafilter.com/271444/Yosemite-2013-laptop-incredibly-slow-performance. It only got better when I upgraded from 4Gb of RAM to 8Gb of RAM, although to be fair I didn't end up trying a number of the other suggestions offered.

Does your Mac have an HDD or an SSD? If it has a hard drive, put your ear to the right hand side of the case when it's beachballing. Do you hear a slow, rhythmic click until it stops beachballing? If that's the case, back up your data immediately - it's probably time for a hard drive replacement. If that's not the issue, I agree with many of the posters - 4GB is too low for a modern Mac. Yes, yes, Apple still technically sells 4GB units, but they're the very cheapest, lowest-end units, and 8GB will likely become the low end later this year. If there's not a problem with the HDD, upgrading to 8GB isn't that expensive on the non-Retina MBPs and will definitely improve performance.

http://osxdaily.com/2014/10/18/clean-install-os-x-yosemite/ are instructions on how to do a clean install of Yosemite. It is not just an update. You will need to back up all of your apps & files, format the hard drive, install the OS, and migrate your stuff back onto the laptop. IMO, this is the route you should take. Your laptop should be running smoothly at 2.5 years old, so there's something wonky going on and a clean install will take care of it (as long as it's not a failing hard drive). Backing up to time machine, doing the clean install, and remigrating from time machine should only take an hour or so of your life.

First version of Yosemite was very slow. First update was much faster than Mavericks. Do it. Be warned, if you have anything installed in /usr/local (like homebrew, npm, or a TeX distribution), move it to another location before installing or the install will take like 12 hours. If you don't know what any of that means you don't have to worry about it.

A three-year-old Mac shouldn't be that slow. My 2010 MacBook Pro is running Yosemite with no unusual beachballing. Do a clean install of Yosemite, and if it's still slow add RAM and consider replacing the HD with an SSD.

Ok, a couple of things... if you haven't been doing occasional maintenance on your Mac, download Onyx. Run it every now and again and have it run its cleaning scripts. It will repair disk permissions, clean caches, etc. Also, I've had no problems with Firefox on a 2.5 GHz i5 with 4GB of RAM. When you download it, go to add ons and install Adblock Edge.

4 gigs of ram is not low for a modern laptop, in fact the Macbook Air https://www.apple.com/uk/macbook-air/specs.html. I have never beeen in a situation when I needed more ram. Does your Macbook Pro have an SSD?

I would personally do an erase and install operation with your MBP first and then try adding RAM. RAM is easy, it's true. You either add/replace it yourself or you get someone else to do it, and when you turn it on the Operating system instantly recognizes and uses it. Erase and install is more difficult and you have to be sure of your backups before you do it. But that said, I think it's more likely to give you long-term results. Honestly, I have an MBP that sounds like it's about as old as yours. It has 16 GB of RAM and an i7 CPU with an SSD drive and it's getting slow. Which is making me think of doing an erase and install. Because I can't add RAM to this sucker. It's already got the max.

Currently running a four year old Macbook Air with Adobe Illustrator, Cinema 4D and the latest Chrome running with 28 tabs open (mainly Ask Metafilter and Pinterest). OS X Yosemite 10.10.1. Running perfectly with no slow-down. Have never clean reinstalled OS X or erased the hard drive. I'd be really interested to know what is causing your slow-down issues as it shouldn't be behaving in the way you describe. Look at Task Manager carefully and see if there are any things running that look suspicious or have names you don't recognise.