Hi Ronny,
Thank you for the details, it helps a lot!
I checked the price slider, but it seems to be all right to me. The most expensive products are for 999 and the cheapest is 0, that reflects on the range slider too.
Sorting options for the front-end filters are unfortunately not possible.
I also tested the performance, and the request were returned within 0.8-1.5 seconds. That seems perfectly fine to me. You can of course fine tune the search configuration, there are many ways to get the best out of it.
The rest depends on the server responsiveness. I highly recommend turning off any plugins which you don’t use – backup/import etc.. as those will affect the site performance in general.