Ego cogito, ergo sum: "I think therefore I am." To play on that famous phrase you could say, "I teach therefore I grade." But who taught you how to grade student work?
Chances are that no professor, faculty orientation or conference presentation provided you with the low-down on how to grade, much less how to grade effectively and efficiently.
If this is the case, you're using Model T grading strategies in a self-driving car grading world.
The good news is that it's relatively simple to learn to implement technology, techniques and strategies that will reduce the amount of time you spend grading.