I like the escalating time, but the pacing issue is a fair point.
So perhaps the escalation could be delayed? Give it a tiny larger timer (let's say, 40s?), and make the second pixel take as much time as the first. Like this:
current times - 30s, 60s, 90s, 120s, 150s, 180s; total 630s
my proposal - 40s, 40s, 80s, 120s, 160s, 200s; total 640s
This way you'd be only getting less pixels per minute after 80s of inactivity, not 30s.