Blake
Hello & Welcome to the Forum!
Your train needs to be run in.. run it around the track in both directions, plus forward and reverse for an hour or so to break the locomotive in. It should run better afterwards.
With any train set, it should be lubed and wheels, pickups & track cleaned before the first use. It doesn't say so in the instructions, but it is a good practice. No telling how long ago the set was produced and had been sitting in a warehouse.
The transformer should supply plenty of power for the loop of track supplied with the set. Yes it will slow down on the curves supplied with the set, you will need to purchase wide curves(O-72) to run warp speed around them. Even then, it may turn into a launch event.
Look through the Maintenance & Repair sub forum here on this site, lots of info there.
Hope this is some help and maybe others will chime in...
Happy Rails!
Dave