A strange improvement

I have an old P3 computer which I run Second Life On. That's not the problem, SL runs reasonably well on it with my souped up RAM and Graphics card. In the Preferences/Graphics the Recommended Settings will set me to Low by default in most of the attributes. This is set this way supposedly to improve the performance of running SL. So then for a long time in spite of this setting I was still getting a lot of lag and slow frame rate. I then decided to test the performance by moving the slider bar to Ultra Quality with only the Draw Distance set to lowest. Then to my surprise the performance improved noticeably. I could move faster and the textures even seemed to download more quickly. So then how can this be? If my computer is a (1) P3 and the recommended settings is for (2) low and the ultra setting is for a (3) P4 computer, then how can it be that the ultra setting has improved my performance?