Realism and combat in D&D and related old-school games is a crazy issue folks drag up now and again and the there is a host of conflicting motives in that realism, one way to have wounds have a realsitic effect is by employing a death spiral to your campaign combats. A death spiral simply put is a method of combat resolution that includes an immediate deterioration in combat capability caused by wounds inflicted. The death-spiral is useful for implementing a sensation of reality beyond the worry casued by lowering HP and it alters the play of grindy HP based combat as well but if there are lots of rules and subsystems involved it increases the time it takes to resolve combat at the table. the following are two simple methods for implementing a deathspiral into your old-school HP based combats.
Method I: Levels Matter
Each time a combatant suffers a wound with damage greater then their level they have suffered a wound. Each such wound suffered causes the combatant to suffer a -1 penalty to hit and damage.
Record the number of wounds suffered, nothing else. Bandaging within 10 minutes will remove 1 wound. Healing magics will remove 1 wound if total healing is over the recipients level. It's possible to continue to suffer the impacts of wounds even when at full HP with this recovery method, this reduces some of the yo-yo effect of healing magic in dungeoncrawls and makes suffering wounds enduring in some cases. I recommend after a week each full day of rest removes one wound (healing magic will get the job done quicker).
High hitdice monsters and high level PCs will only suffer wounds from special attacks or critical hits (if you use them). Fights against multiple opponents will quickly wear down a solo creture/NPC (not a bad thing strictly speaking).
Method II: The Fading Total
As a combatants HP total decreases they weaken. At each 50% reduction impose a -2 penalty to hit rolls and saves (So if a fighter has 20 HP and is reduced to 10 the fighter is -2 to hit and save, -4 at 5 and -8 at 2).
It's best to calculate the different thresholds for the reductions of -2.-4 and so on. High HP combatants will not suffer much from wounds at first unless suffering from a few heavy hits so HP based combat is still going to be grindy for a while but as significant damage is suffered there will be a significant impact on offensive and defensive ability.
Any method that restores HP over the threshold reduces the penalty. when HP aren't very high the yo-yo effect of healing magic will be pretty notable.