sqrt() is a lot cheaper than cos() or sin()

Marlin/src/gcode/motion/G2_G3.cpp

@@ -315,7 +315,10 @@ void plan_arc(
         // Compute exact location by applying transformation matrix from initial radius vector(=-offset).
         // To reduce stuttering, the sin and cos could be computed at different times.
         // For now, compute both at the same time.
-        const float cos_Ti = cos(i * theta_per_segment), sin_Ti = sin(i * theta_per_segment);
+        const float angle = i * theta_per_segment,
+                    cos_Ti = cos(angle),
+                    abs_sin_Ti = SQRT(1 - cos_Ti * cos_Ti),
+                    sin_Ti = angle > M_PI || (angle < 0.0f && angle > -M_PI) ? -abs_sin_Ti : abs_sin_Ti;
         rvec.a = -offset[0] * cos_Ti + offset[1] * sin_Ti;
         rvec.b = -offset[0] * sin_Ti - offset[1] * cos_Ti;
       }