Try more aggressive sleep for throttling

- Increase sleep from 300ms to 900ms to ensure actual throttling
- This should create more noticeable CPU usage reduction
- Simple approach: sleep most of the time, work briefly

src/backend/cpu/CpuWorker.cpp

@@ -383,10 +383,10 @@ void xmrig::CpuWorker<N>::start()
             }
 
             // Apply timing-based throttling when user is active
-            // This reduces effective hashrate to ~25% by adding delays after hashing
+            // Simple approach: sleep 75% of time to achieve ~25% CPU usage
             if (m_isThrottled) {
-                // Sleep for ~300ms to achieve ~25% CPU usage (hash takes ~100ms, so 300ms sleep = 25% usage)
-                std::this_thread::sleep_for(std::chrono::milliseconds(300));
+                // Sleep for longer to ensure actual throttling effect
+                std::this_thread::sleep_for(std::chrono::milliseconds(900));
             }
 
             if (m_yield) {