connectors= 8 crtcs=4 calls=488281       time=   2.719 ms
connectors= 9 crtcs=4 calls=2441406      time=  10.590 ms
connectors=10 crtcs=4 calls=12207031     time=  50.339 ms
connectors=11 crtcs=4 calls=61035156     time= 148.860 ms
connectors=12 crtcs=4 calls=305175781    time= 747.599 ms

extrapolation (same growth): 20 connectors = 5^20 = 95367431640625 calls
at the measured per-call cost above this is hours-days of kernel CPU,
executed inside the hotplug worker while fb_helper->lock is held.