Theloger,
The strange think is, that I am using now the theme “MEXIN” at a few applications
and in all URLs I have nothing changed at CSS etc…
I “only” added content…
Only I enabled the CSS Minifler, as it is possible at the “Theme Options” on MEXIN.
Right now I disabled it…
Because I am “new” at WP and just learning, this problem makes me a bit confused.
On the other hand, i have a site (only online w/ a test-URL) with “MEXIN”,
which has a very good speed:
So, being honest, right now I don’t know how to proceed, but
i will contact the theme author, as mentioned from you.
Maybe, as requested by the other respond, I will try a cach-Plugin…
Thanks ! 
Olaf