If I'm understanding the paper correctly, the bursts had a mean duration of 0.14 seconds, which for a 1000 W microwave would expose you to 140 joules, enough to heat about a shot's worth of water by 1°C. Seems plenty fast to me.
This assumes all the energy is leaked when you open the door, and that the power is constant rather than ramping down. I'm guessing a -lot less- leaks than this.
(And, of course, you don't absorb all of what leaks).
The magnetron itself has about about 65% efficiency, but the paper conjectures that the longer duration of the pulses is due to defects in the cavity that result in some emission at a lower frequency (1.4 rather than the normal 2.4 GHz), so the energy radiated must be a tiny fraction of the nominal power.