After selecting all the files, the CPU usage suddenly spikes when right-clicking the menu.

The delay appearing only when every file is selected suggests Explorer is doing work that scales with the selection, such as querying file properties or invoking a shell extension; the CPU spike alone does not prove corruption. Repeat the test in a new local folder. Compare that with the original and note whether it is OneDrive, network, or mixed content. Restart Windows Explorer from Task Manager and install pending Windows updates. If the delay remains, perform a clean boot and retest. A normal result there points to third-party software. Use Microsoft Sysinternals Autoruns, enable Hide Microsoft Entries, inspect the Explorer tab, and disable non-Microsoft shell extensions one at a time, restarting Explorer after each change. Re-enable items that make no difference. If the delay also occurs in a clean boot, capture explorer.exe activity with Process Explorer or Process Monitor and report the folder type, file count, extensions, and peak CPU.