Here's an experiment you might like to try. Take either a "fast" P2, or any P3 that has an integrated fan, and disconnect its fan from the mobo. Then run it on a mobo that has an adequate temperature sensor, so you're reasonably confident that the reading is more or less sensible. Then, watch this temperature reading go up and up, as you use Windows 9x. Sooner rather than later, it'll get in to the 50s. Next, reconnect its fan to the mobo. Then, watch this temperature reading go down and down. It should drop by very approximately 25C, which is a lot.
Tip: you don't necessarily need a fast fan for slot 1 CPUs. So long as some air is passing through its heatsink "fins" area, that should be sufficient. I have found that the bigger the fan is, the better. Ideally, you want air passing through all of the fins area. For example, lashing a large slow quiet case fan to this area of the CPU works well.