Mekanism-1.12.2-9.4.13.349
forge-1.12.2-14.23.5.2768-universal

Looking at the code here: https://github.com/aidancbrady/Mekanism/blob/1.12/src/main/java/mekanism/common/transmitters/grid/EnergyNetwork.java#L224
the max amount of entries seems to be 6 where the average is probably 2-3, so List should be faster here but there might be a different issue here.

you need to add the extra column in visualvm for number of hits; the time value is useless without it. You also seem to miss the point of a set, if you think it can be replaced with a list

your highlighted line there is 0.111340206185567ms per call. No way to optimise that.