I am with Sinergy.
Two strokes typically provide best power when not yet up to working temperature. so a (10% !!) power loss in air cooled engines when warm is about normal.
This warming up takes about 2 to 3 minutes. Good cooling reduces the loss to a much smaller amount, so the statement seems to be true, that sagging engines are deprived of cooling. Especially so, if the engine is set up a little rich, such that it slowly rises to best power. If an engine set up this way sags in power, it is an alarm phase one!
An engine that is set up lean, will start sagging within a few seconds.
Apart from that, an engine not run in yet, or a tight engine, will not hold full power. The power loss of these engines will occur within less than two minutes, and progress from slight to worse. This calls for immediate action like closing the throttle in order to reduce the load. Failure to do so will lead to progressive friction heat build up and subsequent damage. That is why during the running in, no prolonged full power, or any power level that the engine will not hold, should be asked for.