No, rendering to a vector display (hardware whose primitive operations are points and lines) is almost free for the kind of drawings he was rendering. Zoom is just one linear transformation on each point in the model, no different from panning the view.

I mean, the panning was pretty impressive at the time as well. In Sutherland's 1994 "Bay Area Computer History Perspectives" talk https://www.youtube.com/watch?v=k_cevebLO-A&list=PLKTTWvMgeg... he points out flickering or 'jumping' due to the processor burden of rescaling in a number of places in the Sketchpad demo video, including at 42:30 https://www.youtube.com/watch?v=k_cevebLO-A&list=PLKTTWvMgeg... and at 50:10 https://www.youtube.com/watch?v=k_cevebLO-A&list=PLKTTWvMgeg... .