The spindle on a turning center is either belt-driven or direct-drive. Generally, belt-driven spindles represent older technology. They speed up and slow down at a lower rate than direct-drive spindles, which means cycle times can be longer. If you’re turning small-diameter parts, the time it takes to ramp the spindle from 0 to 6,000 rpm is significant. In fact, it might take twice as long to reach this speed than with a direct-drive spindle.
Source: Buying a Lathe: Spindles and Tailstocks