You might have noticed slightly better performance when installing the iridium plug, but if you did, it probably had more to do with the fact that it was a fresh/new spark plug. There's just not really any performance improvement between a conventional spark plug and a precious metal plug. The precious metal plug just delivers the same designed and expected performance for a much longer period of time.

Performance increases dealing with the spark plug...assuming you have the optimum spark plug for your engine...have more to do with creating a hotter, more powerful spark, and/or a multiple spark system. A hotter, more powerful spark "can" allow a wider plug gap which in many applications "can" improve performance. It can also maintain the spark very robustly in a more hostile combustion chamber environment...like a supercharger, etc., where the spark can be challenged. This usually requires an aftermarket ignition box of some kind, of which there are many. The multiple spark system achieves somewhat the same result but just in a slightly different manner.

Now I'm speaking somewhat in generalities here, as most ignition systems these days are more than sufficient to provide all the spark to the proper plug that the engine can use to provide optimum combustion. It's usually when you start doing performance mods to the engine that an ignition system upgrade/mod "might" provide a performance benefit. Even then most modern ignition systems have more than enough capability to keep up with most standard engine mods like a bigger bore, bigger carb, exhaust, etc. It's more likely that highly increased compression ratios, pressured intakes (supercharged, turbo, etc.), and exotic fuels "could" require a more robust ignition system.

The spark plug just ignites the fuel mixture. As long as it can do so at the right time, with the required intensity, and with the proper operating heat range, you won't get much if any noticeable performance because of a precious metal contact point or not. There are other issues of spark plug operation in a 2-valve head, indexing, and all kinds of neat discussion topics, but they have little application on our lowly KLX.