Andrew, Has your source raster an alpha band ? That could explain the difference since it isn't possible to directly create a YCbCrA JPEG compressed file, but internally a mask band must be created. However I wouldn't anticipate such a huge difference in performance between compression schemes. I would suggest not setting GDAL_CACHEMAX at all and letting it at its 5% default (increasing it is not always a good idea), in case it would be a performance issue at de-allocating cached blocks.
Even -- Spatialys - Geospatial professional services http://www.spatialys.com
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