How do I hide a window without blocking on it?

A customer was working on improving their application startup performance. They found that if their application was launched immediately after a fresh boot, the act of dismissing their splash screen was taking over 5% of their boot time. Their code removed the splash screen by calling Show­Window(hwndSplash, SW_HIDE). They suspect that the splash screen thread has, for some reason, stopped responding to messages, and while an investigation into that avenue was undertaken, a parallel investigation into reducing the cost of hiding the splash screen was also begun.

One of the things they tried was to remove the WS_EX_TOOL­WINDOW style and call ITaskbarList::DeleteTab(hwndSplash) but they found that it wasn't helping.

The reason it wasn't helping is that editing the window style generates WM_STYLE­CHANGING/WM_STYLE­CHANGED messages to the target window, and now you're back where you started.

A better way is to use Show­Window­Async(hwndSplash, SW_HIDE). The -Async version of the Show­Window function is the Send­Notify­Message version of Show­Window: If the window belongs to another thread group, then it schedules the operation but does not wait for it to complete.