Programs on your computer are nothing more than bits of data, 1s and 0s. When they are sitting on your computer they do not affect how fast you browse the web, or play a game, or watch a movie, or how fast your computer starts up.
You can think of your hard drive as a filing cabinet just storing your programs and files for use at a later time. When you need to use a program or pull up a file your computer knows the precise location of the program/file and retrieves the file or loads the programe extremely quickly.
Simply adding more programs to the hard drive doesn’t make your computer run slower.
Obviously should you run many programs at once this could have an effect on the response time and overall speed at which your PC runs, depending on the amount of Ram dedicated to each specific program as well as the CPU utilization of such program.
There are obviously a finite amount of programs that can be run at any one time without you experiencing a "slow down" however, I doubt that most if any users would, with a decent PC, have enough programs open to seriously affect the PC's operation.
Hope this helps.