Your 671 might be in need of servicing.
As Chuck pointed out, the only reason a 671 might be slower is if it were the 1946 model with TWO worm gears and a driveshaft running fore-and-aft in the chassis. The '46 model also has a "smoke bulb," which was replaced by a wound-wire smoke element when Lionel redesigned the loco the following year. After 1947, the only other changes were the addition of Magne-traction in 1950, changing the number to 681. Later in the '50s, a lubricator linkage was added to make the 682. It sounds like yours is one of the redesigned 671s (a photo would confirm this.) So if it's properly serviced and not worn out, it should run about the same as your 681, except with less traction and cornering grip.
One common problem with Turbines and Berks is that the motor is geared to the last axle. Over time, the torque causes the rear axle bearings to wear in an oval shape. This allows the rear wheel to rub on the chassis. If it gets bad enough, the loco will hesitate, or not run at all. Unfortunately, replacing the axle bearings is an expert-level repair best accomplished with wheel cups and an arbor press. Lacking those, some people insert one or more plastic clips from loaves of bread between the worm wheel and the inside of the chassis to keep the axle centered, temporarily restoring smooth operation. Good luck!
