I spent a while compiling some pypy-c source with different optimization options and running pystone (on an iMac with a 1.8 ghz G5 and 1.5 gigs of ram). Here's a table of results: 0 1 2 3 3p s sp c 0 1.00 3.06 3.08 3.08 3.17 3.12 3.25 45.39 1 0.33 1.00 1.01 1.01 1.04 1.02 1.06 14.82 2 0.32 0.99 1.00 1.00 1.03 1.01 1.05 14.72 3 0.32 0.99 1.00 1.00 1.03 1.01 1.05 14.74 3p 0.32 0.97 0.97 0.97 1.00 0.99 1.02 14.31 s 0.32 0.98 0.99 0.99 1.02 1.00 1.04 14.53 sp 0.31 0.94 0.95 0.95 0.98 0.96 1.00 13.98 c 0.02 0.07 0.07 0.07 0.07 0.07 0.07 1.00 Here's a key: 0 : -O0, aka "no optimisation" 1 : -O1 2 : -O2 3 : -O3 3p: -O3 -fbranch-probabilities (i compiled with -fprofile-arcs and ran pystone to generate the branch info) s : -Os. aka "optimize for size" sp: -Os -fbranch-probabilities c : cpython for reference As you can see, you definitely want *some* optimization but it doesn't make a great deal of difference what level you choose. That said, -Os gives the best results and -fbranch-probabilities gives a ~5% gain. I timed each compilation, but forgot to write the results down anywhere. There weren't vast differences at any rate (they all took ~7 minutes, not too bad). Of course, using -fbranch-probabilities requires you compile everything twice. The machine wasn't totally quiet as I ran these tests, but fairly. Cheers, mwh -- What the semicolon's anxious supporters fret about is the tendency of contemporary writers to use a dash instead of a semicolon and thus precipitate the end of the world. -- Lynne Truss, "Eats, Shoots & Leaves"