Under a low load the iR is less of a factor, but the inefficient buck/boost circuit is. The higher quality (expensive) power banks use much more efficient (read:optimized) electronics. I have a power pack for a MagicShine bike light that I use (2S2P 21700s) and it only does USB C PD up to 23 W, but it performs so much better than the cheap Chinese ones (but cost about $200).
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