How load large volume assignments in OTL/AM without PPM?

What do HCM Time and Absence customers who do not have PPM do to load assignments to an employee timecard?
We will have likely millions of Project/Task codes across when we fully implement HCM OTL/AM. We've heard PPM is the solution to make this work without impact to performance among other things. If your volume is also this large, and you do not use PPM, how do you load assignments today? And what is the UX like both from an enterprise level and an employee level?