In lisp how can I measure and capture the time spent evaluating an expression?

I want to capture the results of a call to the time macro in order to gather multiple measurements and process them. I tried to locally setf the standard output and redirect it to a string but it didn't work with the time macro. Maybe it is wrong, but what I tried is:

(with-output-to-string (str)
    (let ((*standard-output* str))
        (time (test-with-size 40))))

The questions:

1. Is a way to capture the output of time?
2. If not, can I capture the slime-profile-report command's output?
3. If none of the above works, how can I measure the time spent evaluating an arbitrary expression?

What I want to accomplish is to measure the run-time of an algorithm as the size of the input increases so for each input size (ranging from 1 to 100) I will measure a lot of times and keep the average. Then I want to plot the results. Plotting is easy, and I have found many ways in Cliki, but how can I gather the results?

I am using CLISP and CCL.

EDIT: Paul Nathan pointed that the time macro outputs to *trace-output* which is a solution. I would like a nicer, simpler, solution though, because with this one, I have to parse an implementation specific trace.