In my experience Shaxpir generally starts out very responsive to key presses, but as the chapter/note/etc… you’re working within grows larger and larger, the responsiveness slows a bit. Never really bothered me when typing, but when holding down backspace I would often delete far more than intended due to the slow refresh rate.
Can you tell me how many words the chapters you’re working in are when you start to notice this problem? I’m usually good up until about a thousand, at which point I start to notice the slightest lag, but it’s not until four to five thousand that it really starts to affect me.
I can’t comment beyond that because I don’t write short stories, and because I limit my chapter sizes I never really exceed this count. But if it is word count related and if I were writing short stories (to the tune of ten thousand words) or novellas (twenty-five thousand words) then I could absolutely see it having a big impact.