All INAV versions suffer from slow refresh rate when using dji WTFOS osd. Rapid high amp changes is good indicator of bad PID/filter tune. With INAV osd amp meter is useless because of slow update interval. On average it looks like 3Hz.
Changing osd_msp_displayport_fullframe_interval makes no difference.
With betaflight refresh rate is way higher with the same default settings of the WTFOS.