Thank you, guys, for your answers.
Firstly, this is an opinion of one guy whose proficiency in comps I don't know. I had some vague apprehensions about this 'the more browsers the slower your system' issue based on my own experience.
Secondly, I believe I can consider your opinions as quite expert. So we can summarize that the opinion 'the more browsers you have (even if you don't start therm) the slower your comp works' is wrong.