Welcome to the forum!

It is natural for the no-load voltage of a transformer to be higher at a specific throttle setting and to be lower under load at the same throttle setting.

You may need to service the locomotive - clean and oil the motor bearings, grease the gears and oil the wheel/axle bearings. Oil the wheel/axle bearings of the rolling stock also.
Clean all the wheels and roller pickups with Naphtha. Clean the track with Naphtha also.
Make sure the track pins are clean and free of dirt and rust, and are a tight fit in the tracks.

Trains in need of lubrication will run slower and need more voltage to run.

Labelle lubricants are ideal for our trains.
https://labelle-lubricants.com/shop/kit-with-one-each-102106-and-107-for-ho-o-s-lionel-and-lgb-garden-railway-size-trains/

Larry