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Some of the early V3 code was not as efficient as it should have been - the Windows message pump was working too hard. Messages have been replaced with callacks and high-precision timers, as a result performance and CPU load on lower-powered computers such as the Core2Duo and mobile CPUs has improved, sometimes quite dramatically.
Select from the Defns panel in the ribbon bar, Defns is only shown if you have enabled one or more definitions groups.
In the Organiser select the Definitions, click … to define groups (top of window, see below).