Reduce Step Smoothing ceiling to 50% CPU usage (#18719)

Marlin/src/module/stepper.cpp

       #if ENABLED(ADAPTIVE_STEP_SMOOTHING)
         uint8_t oversampling = 0;                           // Assume no axis smoothing (via oversampling)
         // Decide if axis smoothing is possible
-        uint32_t max_rate = current_block->nominal_rate;    // Get the maximum rate (maximum event speed)
+        uint32_t max_rate = current_block->nominal_rate;    // Get the step event rate
         while (max_rate < MIN_STEP_ISR_FREQUENCY) {         // As long as more ISRs are possible...
           max_rate <<= 1;                                   // Try to double the rate
-          if (max_rate >= MAX_STEP_ISR_FREQUENCY_1X) break; // Don't exceed the estimated ISR limit
-          ++oversampling;                                   // Increase the oversampling (used for left-shift)
+          if (max_rate < MIN_STEP_ISR_FREQUENCY)            // Don't exceed the estimated ISR limit
+            ++oversampling;                                 // Increase the oversampling (used for left-shift)
         }
         oversampling_factor = oversampling;                 // For all timer interval calculations
       #else

Marlin/src/module/stepper.h

 #define MAX_STEP_ISR_FREQUENCY_2X   ((F_CPU) / ISR_EXECUTION_CYCLES(2))
 #define MAX_STEP_ISR_FREQUENCY_1X   ((F_CPU) / ISR_EXECUTION_CYCLES(1))
 
-// The minimum allowable frequency for step smoothing will be 1/10 of the maximum nominal frequency (in Hz)
-#define MIN_STEP_ISR_FREQUENCY MAX_STEP_ISR_FREQUENCY_1X
+// The minimum step ISR rate used by ADAPTIVE_STEP_SMOOTHING to target 50% CPU usage
+// This does not account for the possibility of multi-stepping.
+// Perhaps DISABLE_MULTI_STEPPING should be required with ADAPTIVE_STEP_SMOOTHING.
+#define MIN_STEP_ISR_FREQUENCY (MAX_STEP_ISR_FREQUENCY_1X / 2)
 
 //
 // Stepper class definition