divide only when required

Marlin/src/module/planner.cpp

@@ -1092,6 +1092,7 @@ void Planner::_buffer_steps(const int32_t (&target)[XYZE], float fr_mm_s, const
   float max_stepper_speed = 0, min_axis_accel_ratio = 1; // ratio < 1 means acceleration ramp needed
   LOOP_XYZE(i) {
     const float cs = FABS((current_speed[i] = delta_mm[i] * inverse_secs));
+    if (cs >  max_jerk[i]) 
       NOMORE(min_axis_accel_ratio, max_jerk[i] / cs);
     NOLESS(max_stepper_speed, cs);
     #if ENABLED(DISTINCT_E_FACTORS)