Backend very slow when logged in via mapped domain

Hello,

I have the problem, that the backend of one of my sites loads really slow, when I enter it via the mapped domain.

When I enter it via the original domain, it takes 3 seconds to load.

When I enter it via the mapped domain, it takes 13 seconds to load.

Do you know what the problem could be?

the mapped domain is http://www.lights.de

Thanks