It would be better to search the user based on role while Masquerading. It will major impact on performance while user data size huge.
Closed: won't fix
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There are others types of access besides role, so there's no way to guarantee the result. The goal of the module is to view the page as a specific user. I manage a site that doesn't use only roles for access, so if masquerading was by role, it would not work.