New in 1.5, StringBuilder is a drop in replacement for StringBuffer but is
supposed to be faster due to being unsynchronised, it certainly will not be
slower.

There are a small number of places, certainly in 2D code and perhaps across
other class libraries we should make this replacement as at least some of these
uses affect start-up time.
(The compiler is already being updated to use StringBuilder for "+" style
string concatenations).
The 2D ones that sprint to mind are on Solaris in X11GraphicsEnvironment
and MFontConfiguration. The latter reads a properties file and whilst what's
probably needed is to rework that entirely there may also be some benefit
from java.util.Properties using StringBuilder internally (it currently
uses StringBuffer).

It may be "noise" in terms of start-up performance but the risk is low and
the change simple.