I recently wrote this response to a family member who asked about recommending a SSD to put in a late-2011 MacBook Pro.
An SSD is easily the best upgrade for performance of an older MacBook Pro. I recommend the Samsung 850 EVO SSD. They have the best performance and lowest power consumption of any SSD that I have tried. You can get them in 250GB, 500GB, and 1000GB. Other sizes are available, but these are the most common. There is a “850 Pro” that costs a bit more than the EVO but you'll probably never notice the difference in performance – at least not enough to justify the cost difference.
One thing you'll need is a program that enables TRIM support for the SSD. It keeps your SSD blocks clean, extends its life, and keeps performance up. More info here: https://en.wikipedia.org/wiki/Trim_(computing)
OSX automatically handles this for Apple-supplied SSD's, but not for 3rd-party ones. I use a program called Disk Sensei to handle this. It's about $20: https://www.cindori.org/software/disksensei/
You'll also get a huge performance increase by putting in 8GB of RAM. You just need to make sure it's “Mac memory”. Corsair is really good, and they have a website that you can check compatibility: http://www.corsair.com/en-ca/memory-finder
Best places to buy components are NCIX and Memory Express. Amazon can sometimes come close in pricing.
iFixIt will provide you with free (and awesome) step-by-step guides to upgrade both of these. https://www.ifixit.com/Device/MacBook_Pro_13%22_Unibody_Late_2011 (assuming you have the 13″ laptop)
Question: What tech device do you use regularly that doesn't perform quite as well as when you first started using it?