Boost PFC controller with fast transient load capability?

Install a brownout capacitor close to the output stage. Arrange a switching circuit so it charges (from the output stage) to your desired threshold voltage while everything is running normally. Prevent the cap from discharging and allow it to hold its voltage in reserve.

Then as soon as you detect a voltage droop, switch the cap over instantly so it substitutes for the output stage. This gives time for the control circuit to resume production.

A desired reference voltage is not readily available in the boost converter. The control circuit needs to build its voltage, then self-regulate the output. It's difficult to make a feedback circuit which has high gain to correct its voltage, yet never overshoots the target value. The blackout capacitor always has the target value at the ready.

Usually this is implemented as a separate analog circuit - that puts a defined short signal into the main control loop - to reduce volt sag under sudden load application,

we were designing such " speed up " ckts way back in 1992 to go with the common UPF IC's of the time, e.g. UC3854, UC3854A, and a myriad of others.

also useful to perform the complement - that is, reduce pwm to zero as the Vout approaches a maximum tolerable on the main bus caps.

Usually this is implemented as a separate analog circuit

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Thanks, i see what you mean, yes, the offtheshelf ones only have a weak speed up , if at all, by the looks of things.