I'm not sure what to say/do about this.
First, why should someone run with full logging enabled if not inside a deep debug session...
Second, it doesn't surprises me full logging slows down rendering... that's what it does after all with all that string serialization and concatenation.
Even though, I think the actual problem is the log4j->java,util.logging level mapping.
That is, setting a DEBUG level on the log4j properties file enables FINE, FINER and FINEST on geotools using java logging.
Took a deep look at the shapefile renderer logging statements and they make quite good sense to me, like the more annoying stuff is being logged in FINEST.
So what the solution could be? we of course could limit the org.geotools.renderer.shape logger to a higher one than DEBUG for the GEOTOOLS_DEVELOPER profile, but.. that means loosing FINE and FINER too, aka, CONFIG and higher?