I have the impression MT API is queried every time one moves from one segment to another, if the MT engine is loaded and not set to "Stop Machine Translation".
If that is correct, and although there is always the possibility to select "Stop", I suggest to have the MT result cached within the same session, to avoid unnecessary queries.
Also, it could be usefuly to add an option in Preferences or per MT engine (context menu) to enable MT only for "untranslated" segments (i.e. empty segments).
When I'm reviewing/QA-ing my project, there's no need that any segment is sent to any MT anymore.
The feature is available starting from update 12 of the Yeddi release.