You appear to be mixing up CPU usage and Load. in particular your example of 0.15 CPU * 20 Users = 3 Load doesn't make sense as 0.15 is a percentage and load is absolute

Load is the number of processes that are waiting for time on the CPU. That would be the 10 or 15

% CPU is the amount of real time that the process spent actually running eg 0.15.

So in your example if your script was taking 0.15 CPU over 1 second and you had 20 users running it simultaneously you would be trying to use 20 * 0.15 = 3.00 CPU which means that you will be using 100% of the CPU for 3 seconds. and that would translate into roughly 20 load at 0 seconds 14 load at 1 second 7 load at 2 seconds and 0 load at 3 seconds. Assuming each request ran to completion 1 after the other. Which they probably won't what with the vagaries of I/O and scheduling.

And of course if you have a multi proc box then the real time taken is % CPU / # of CPU's

I'm not sure if the %CPU time is the percentage the process uses over a fixed length of time eg 1 second. or if it's the percentage used over the length of time it took to run to completion.

Anyway I hope that explains the difference between Load and CPU usage ( and I hope it's reasonably accurate). I also hope it helps you find the problem, and of course as other ppl have already said, Do a actual test, *this* is just to help you interpret the numbers a bit better :). Also Load is usually averaged over the last minute 5 minutes and 15 minutes