ThinkPad

Then you should consider replacing your existing HDD with an SSD(solid state drive)

Upgrading the memory in your computer is usually the first step to increasing it's performance. Additional memory allows you to run more applications simultaneously without the computer slowing down. It also allows you to multi-task to a much greater degree as your applications, email, web browsing, etc. all have more RAM to use to run efficiently.

However additional RAM does not solve all the issues of a "slow" computer. If you find that it takes the latest OS a long time to load or your spreadsheets are taking minutes before they are ready to use, then you should consider swapping out your old spinning HDD for an SSD.  

SSDs will run up to 12 times faster then a HDD and will have the OS loading and ready to use within seconds instead of minutes. All complex programs, high res video, complex audio and other large applications will load, ready to use, within seconds instead of taking several minutes before being ready to utilise.  

In a working environment, waiting minutes each time a new program loads or for your computer to be ready to use, is a time consuming and costly exercise. Added up, wait times increase to hours lost when totaled over the course of just a week. 

None of us here have that kind of time to waste, do you?