The reason may be that Windows uses a graphics card to speed up the drawing processes in the Aero tag.
Therefore, if you deactivate the Aero theme, all processing is performed by the CPU (instead of the GPU), which leads to a decrease in performance compared to the hardware accelerated task (if it is performed by the GPU). The non-Aero theme uses only a processor for drawing processes!
The source of my claim can be found here.
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