I remember using "segments" in AD&D combat rounds, especially as it came to casting times. I don't remember if we ever used it right, but if you are going to use spell interruption in your campaign, it's kinda necessary.
I also remember that we slowly forgot to use casting times, and when we put that to the side, segments transformed into initiative order and little more. Of course, as casting times went to the side, so did casters loosing spells to interruption. I'm sure it change the game balance, but we didnt care, as it sped up play.
It went both ways. Getting the drop on an enemy caster was no longer as effective, as his spells were no longer getting interrupted either.
Maybe it was one of out earlier house rules, along with ignoring weapon speeds (as we could never make hide nor hair of it, and we tried) and weapon vs armor type adjustments (because we wanted it to apply to monsters and humanoids too, but as written it didnt.)
The Minotaur for Old-School Essentials
*Requirements:* Minimum STR 9, Minimum CON 9
*Prime Requisite:* STR and CON
*Hit Dice:* 1d10
*Maximum Level:* 8
*Armor:* Leather, including shield...