Soldering

make sure the tip of the iron is very clean, if it's dirty the dirt will absorb the heat.  To clean it all you have to do is heat it slightly, not full temp, and use a scraper or razor and keep scraping it until shines again.  Other tools could be sand paper or steel wool.  Sometimes they work better.  Otherwise that should help you out. Is by chance those steps don't help it is probably time to replace the tip.  As it wears out it will stop heating the wires quickly.  You will be greatly surprised at how much of a diference changing the tip or cleaning it will make.