Do you have an old computer?
"Old" is a relative term, Sol. I have (and use) a computer that has 512Mb of RAM and a Pentium CPU. It runs this site just fine and the editing is just as rapid as my newest 64-bit speedster gaming box. In a lot of cases, the speed at which editing (or other in-browser activities) is handled isn't the computer itself, but the browser, which should have been your question.
I rather suspect that you editing box may have quite a bit of format-checking done on each keystroke that adds a delay to the character as it is entered and then displayed. Further, I've noticed that highlighting a phrase in the editor and then altering the contents takes a finite amount of time.
Tom