Mostly use unorthodox variable-width font for coding, but do need monospace sometimes, e.g. for jagged YAML files or .rst.
Had weird issue with my emacs for a while, where switching to monospace font will slow window/frame rendering significantly, to a noticeable degree, having stuff blink and lag, making e.g. holding key to move cursor impossible, etc.
Usual profiling showed that it's an actual rendering via C code, so kinda hoped that it'd go away in one of minor releases, but nope - turned out to be the dumbest thing in ~/.emacs:
(set-face-font 'fixed-pitch "DejaVu Sans Mono-7.5")
That one line is what slows stuff down to a crawl in monospace ("fixed-pitch") configuration, just due to non-integer font size, apparently.
Probably not emacs' fault either, just xft or some other lower-level rendering lib, and a surprising little quirk that can affect high-level app experience a lot.
Changing font size there to 8 or 9 gets rid of the issue. Oh well...