Actually, you should be able to play runs with equal speed either picked or by hammering on. Economy of movement is whats important when it comes to picking and being able to play smoothly on 1,2,3 and 4 note per string runs. The main difference between picking every third or fourth note is the sound. A picked note has a very definite "picking" sound which should clearly separate each note in the run whereas a run comprising hammer-ons sound more fluid. Each has their place in any piece of lead, learning to play both properly will add a lot more flavour and dynamic to your playing though. Also, if you can play a long impressive run using hammer onsand then follow it up with an even more technical lick that picks every note, you will find it easier to grab the listeners attention.