In WOTLK classic the latency measurement doesn't seem to work properly. Not sure if it's the same on retail.

Looking at the code, the latency is calculated using the difference between CURRENT_SPELL_CAST_CHANGED and UNIT_SPELLCAST_START. The UNIT_SPELLCAST_SENT is commented out and unused, I tried uncommenting it and it's never called.

Adding some print statements to the hooks I can see CURRENT_SPELL_CAST_CHANGED event is called many times per auto-attack, spell cast as well as pet spell cast. Additionally, macros with many different actions (like startattack, cast, petattack, etc.) compound this issue. The result is that the latency measurement frequently does not display (since it's 0, due to the changed event happening at the same time as the changed event), and when it does show it is likely wrong because it's using the last change event time.