I have a 2010 Mac Book Pro (MBP) that I'd upgraded for every new release of OSX. Never had I done a fresh install until late last year. While it did help it's performance it didn't make a huge improvement. The PC was still sluggish.
A little bit of history. After only a short length of ownership I'd previously upgraded the ram to 8GB from 4GB (thanks to OWC). It had a fast for it's time i7 2.66 GHz processor and a 7200 rpm 500GB hard drive. At the time I bought it, I'd never had a faster PC.
When I started looking at prices for a new Mac Book Pro I couldn't justify spending almost $4000 for less storage then I currently had (256GB). Even more so when the current i7 chips in the MBP's are over 2 years old. We still seem to be hanging out for Skylake i7's.
So I started researching SSD drives and found the Crucial BX200 offered good value and performance from PBTech clocking in at 480GB for NZ$250. SSD storage has never been as cheap at under $2 per GB mark. Importantly the Crucial drive includes a spacer to pad out the drive in the Mac Books body.
I'd previously got a screwdriver to fit the Mac from the OWC ram upgrade a Phillips #00. You will also need a T6 Torx screwdriver also, I got mine from Bunnings for $6. Follow the guide on the Crucial support site as it walks you through step by step with images. It's a easy install taking less than 30 minutes.
Once installed I suggest you do a clean install of El Capitan. I followed the Mashable guide and created a USB installer.
After a month the drive has given my MBP a new lease on life. It's faster than it's ever been, and the battery lasts longer now. Getting another hour, for a total of 5 hours compared to 4 previously.
If you have an old Mac without an SSD I'd recommend you install one, well worth the upgrade price.