How software makes computer slow?

This is a very hard question to answer because there are a lot of factors that can contribute to a computer running slow. I'm not a computer technician but in my estimation from my experience with computers the most likely reasons would be:

1. Spywares or Viruses

2. Programs running in the background eating up your computer resources.

3. You are running programs too heavy for your computers capabilities, thus overloading it and causing it to run slow.

In 90% of the cases its Spywares or some Virus. You can sometimes spot if you have a spyware eating your memory from the Task Manager and closely surveying what programs are eating the most resources and if they are indeed something you yourself have installed or something unknown.