The XSL: Speed Optimizer section (screenshot below) enables you to define options for XSL Speed Optimizer.
A time threshold for single XSLT instructions in an XSLT stylesheet can be specified for the Optimizer. Values range from 0.1% of total transformation time to 99% of total time. If an instruction takes more time to execute than that specified as the threshold, then optimization analysis is invoked. Otherwise no analysis is carried out. If optimization analysis is unsuccessful, the reason might be that the time threshold is too high. Consider lowering it.
After making the settings, click OK to finish.