You can use StopWatch >> or the native time counter for the language you are coding on.
Alternatively, you can use the aqPerformance object in scripts (equivalent is the Performance options in keyword tests). These do pretty well as well.
Keep in mind that there is overhead of TestComplete's processing. Depending upon what you have wrapped within the timer checks, there may be object recognition, script parsing/execution, etc., that may add to the time. So... while it will be a good timing, it won't be a fully accurate timing of the actual load time for the dialog. Just keep that in mind as you're taking your measurements.
