On the C64 I will change the border color when my frame routine starts and change it back once my routine finishes. This will tell me how much of a fraction of the total frame time I use, in quite literal terms. I wonder if similar tricks were used for VGA. I think you could change color index 0 to the same effect.
Speaking about the C64 the exact instruction timing was also key to the two rather clever hacks to put sprites outside the nominally drawable 320x200 pixel screen area.
First in the top and bottom border. This was done by waiting for the GPU to start drawing the bottom line of text on the screen and then switching to a mode with one less line of text. The sprite hardware would then not get the signal to stop drawing after the last line since that presumably already happened with fewer lines to display. This would cause it to keep drawing sprites below the bottom border. The in the vertical blanking before the top line was draw you would switch it back from 24 to 25 lines of text.
The side-border is a variation on this where you time your code to wait for the 39th character of a scan line then switch from 40-character wide to 38-wide. Again the sprite engine would not get the signal to stop drawing and continue drawing in the side border outside the 320x200 pixel nominal capabilities of the hardware.
For side border it was necessary to do this for every scan line (and every 8th line would have different timing, probably related to fetching the next line of text), so timing was critical to the exact CPU cycle.
These machines were not much by modern standards but for the hacker mind they were an amazing playground where you had just your assembly code and the hardware to exploit without the layers and layers of abstractions of modern gear.
I did that for A game in DOS days. It needed a virtual retrace interrupt using a timer interrupt to trigger shortly before retrace, busy waiting until retrace then recalibrating the timer for the next frame.
Pretty soon after that Operating Systems managed memory and interrupts themselves. The added latency on timers made precision tricks like that impractical.
