Does 18650 cell voltage sag have a direct relationship with its internal resistance?
Yes, that's the basic effect of the ESR (equivalent series resistance) and the capacitance of the cell.

If you use any free circuit simulator and play around with reasonable values of R and C, you will get a better understanding of what's going on inside the cell. If you measure the given voltage drop with a known current and the time taken to recover, you can derive the correct values of Rs, Rp and Cp in the example below.