Not just timing critical stuff. BIOS calls for serial were slow, character at a time. If you wanted to fill a buffer and keep processing you wrote your own interrupt driven serial code or used a good library. No serious software on DOS would use the BIOS serial code.

This sort of thing was just normal. No multitasking so you had full control of the hardware. Third party libraries or custom code for serial, sound, graphics, often keyboard. Drop into assembly for extra speed. DOS was basically a library for filesystem access, process creation and limited memory management!

I still have 1.193180Mhz burned in my brain...