I have written a chess engine in C++ which uses a single thread at 99% CPU busy. In November and previously, a test position from the initial set up, looking ahead 17 moves, used 20.9 seconds of CPU time consistently under Windows 10. Now the same calculation uses 63.5 seconds consistently when no other major tasks are using the CPU. The thread is still running at 99+% and the same number of chess positions are evaluated as before ( 50,021,849 ). The CPU still gets 3.6 to 4.1 GHz. Windows 10 has updated to 1903 which may be a coincidence, since I have turned off all apps using significant CPU time. Nothing else shows up on the monitors, and my power settings are high performance and sleep NEVER. The CPU just seems to take 2 - 3 times as much time to process the same program as before. Is it hardware or software at fault? The BIOS is from HP as is the hardware. 8 GB ram and 500 GB SSD. No bottlenecks except CPU busy.