Yes. In continuous shooting, burst rate can drop if the camera is waiting on autofocus, especially in AI Servo. A slower-focusing lens can make this more noticeable.

The key point is that continuous drive speed is not determined only by shutter speed. In AF modes that track focus, the camera may delay frames while it evaluates focus and drives the lens. With AI Servo, the first frame is typically release-priority, but subsequent frames can be influenced more by focus-tracking behavior, so the lens’ focus speed can affect the practical frame rate.

That explains why a USM lens may feel faster in bursts than the EF 50mm f/1.8 II, whose focus motor is comparatively slower.

If you want maximum burst rate, options include:

• using release-priority settings if your camera offers them,
• prefocusing or switching to manual focus,
• using a faster AF lens,
• or using a different AF mode when subject movement allows.

So: you’re not imagining it — autofocus performance, and specifically lens focus speed in AI Servo, can affect real-world continuous shooting speed.