LattePanda Sigma performance hit due to undervoltage

I'm working on some plans for an upcoming project with a LattePanda Sigma, and I want to build my own battery for it, but I am concerned about not getting the right voltage for it. I know the supported range for it is DC 12-20v, and I would probably be going with 18650 for the cells.

But there is no way to cleanly hit 20v with 3.7v cells. If I do a 5s configuration, I'd be 1.5 volts under the upper guidelines. Is that going to have any noticeable performance impact?

I've also been thinking about maybe doing something far higher and stepping down the voltage, but I'm not sure if it is worth the hassle.