Download a free program like Autoruns. This will show you what programs are starting with your computer. The more things that are starting, the slower it’s going to run.
Use Autoruns to disable everything that isn’t essential (hint: security software is essential). You should see a nice bump in speed. If you find you’re missing something that needs to start, you can re-enable it. One thing that can slow your system down is someone stealing your Internet. The bonus Q&A below will tell you how to spot an Internet thief.