I kept getting low voltage warning with a beefy 3 A universal connector, adjustable voltage radio shack (remember that place?) wall-wart supply I rigged directly to the pins to power stuff including a pi, and save the pi USB port some plug/unplug cycles, while set to 5 volts. It has switchable like 5, 6, 9, 12, 18 V settings or something.  This is my "test setup".

The Pi had subpar performance on what seemed like tasks my old Pi 3B+ from 2015-2017 should be capable of.

I was hesitant to try kicking up (*all the way!<gasp>*) to 6 V with these tight specs listed here (and on the mfg website), but knew there must be at least 0.5 V drop on my 4-6' USB replacement wire, yet it was reading ~4.95 V open circuit (no load). 

Since my former employer wanted my ***calibrated*** and extremely accurate Fluke meter back (read: calibrated), my chinesium Harbor Freight Tools meter is maybe +/- 0.5 V on a good day, and even the $100 "premium" HFT meter is +/- 0.1-0.2 V depending on where on the nonlinear curve you are (not accurate or precise and ***completely uncalibrated***), why bother setting up a complex fixture to try and measure voltage at the pins under various CPU load when the chinesium pi jumpers will cut all voltage as soon as you start separating them from the pins, changing the load conditions with an inadvertent forced reboot and invalidating the test, for a $35 SoC.  So we continue to speculate about theoretical voltages in this thread.

One day got mad at the Pi, and the project was likely to damage the pi anyway, so kicked it up to 6 V! insane! ... it performs much better! Learned the RPi definitely has a "throttling" mode where it throttles the CPU, even triggered the low voltage warning a few times on 6 V, but performance is much improved since it's usually not throttling now.

**TL;DR:** if your Pi is lagging, or you're getting low voltage warnings non-stop, check for CPU throttling or don't fear **MOAR VOLTS** (within reason) and just go for it until the symptom resolves, so the current supplied while actually using it with a load doesn't voltage sag even a decent wall-wart (for you CSE types) -- these datasheet numbers cited here are academic theory for pencil pushers and were meaningless in practice! $35 RPi on to the next project. Christmas is coming! Get a bench PSU or at least an adjustable wall-wart if you're enough of a tinkerer to have a RPi. Sorry StackExchange, the personal experience was right in this case, and this is based on more than opinion. (BS, MS EE -- surprisingly fun)