According to the javadocs, the different text-rendering values have the following effects:

'optimizeSpeed':

• KEY_RENDERING=VALUE_RENDER_SPEED
• KEY_ANTIALIASING=VALUE_ANTIALIAS_OFF
• KEY_TEXT_ANTIALIASING=VALUE_TEXT_ANTIALIAS_OFF
• KEY_FRACTIONALMETRICS=VALUE_FRACTIONALMETRICS_OFF

'optimizeLegibility':

• KEY_RENDERING=VALUE_RENDER_QUALITY
• KEY_ANTIALIASING=VALUE_ANTIALIAS_ON
• KEY_TEXT_ANTIALIASING=VALUE_TEXT_ANTIALIAS_ON
• KEY_FRACTIONALMETRICS=VALUE_FRACTIONALMETRICS_OFF

'geometricPrecision':

• KEY_RENDERING=VALUE_RENDER_QUALITY
• KEY_ANTIALIASING=VALUE_ANTIALIAS_DEFAULT
• KEY_TEXT_ANTIALIASING=VALUE_TEXT_ANTIALIAS_DEFAULT
• KEY_FRACTIONALMETRICS=VALUE_FRACTIONALMETRICS_ON

However that does not match the actual behaviour: In optimizeLegibility, KEY_TEXT_ANTIALIASING is set to VALUE_TEXT_ANTIALIAS_OFF, and geometricPrecision sets both KEY_ANTIALIASING and KEY_TEXT_ANTIALIASING to ON.

The best strategy is probably to set KEY_TEXT_ANTIALIASING to VALUE_TEXT_ANTIALIAS_ON in optimizeLegibility and change the javadoc for geometricPrecision to reflect the current behaviour.