a few things to check, or change on the beater bike
do the wheels spin free? hub and crank bearings in good condition?
are the brakes dragging? narrow slick tires at a decent PSI? or cheap low pressure heavy fat slicks ?
I used a mid level 26" mountain bike as my general purpose bike for decades
everything from a quick trip to the store, to full century rides (100+ miles)
the only major change I made to the bike was a pair of decent 1.5 X 26 high pressure tires
and a good seat, not as fast as one of my road bikes, but much more comfortable