There are a lot of issues with using Google Analytics for collecting performance data. As mentioned, their use of averages leads to a lot of problems and issues from outliers. GA heavily samples (1% by default) & only collects timing data from browsers that support Nav Timing. If you're looking for a way to collect more accurate timing data, Torbit Insight (http://torbit.com) is free and offers 100% sampling, histograms, medians, percentiles, and lots of breakdowns by browser, geography, etc.