The entire transaction, according to the spec, takes ~400 ns at 26 MHz. Traditional I2C would take 4-5 µs. That 10x speed difference allows for aggressive dynamic voltage and frequency scaling (DVFS), saving significant battery power.
When no power management adjustments are needed, the masters can transition the SPMI bus into a low-power sleep state. The SCLK line stops toggling, eliminating dynamic switching losses. mipi spmi specification pdf
Staying current requires monitoring the MIPI website for specification updates. Always re-download the latest when starting a new project. The entire transaction, according to the spec, takes