Lightroom Develop Module does not utilize the Previews. The previews are used when browsing in the Library Module.
The develop module has to render the original file and whatever develop work is associated with the file.
1:1 Previews are utilized in the library module. In the develop module, data has to be loaded from the catalog to ensure that you are seeing all of the adjustments you have made.
As DdeGannes and Jim Hess already said: "that's normal" - it's how Lightroom works, for everybody.
It is after all a non-destructive editor, which means there is an image which never changes, and a set of adjustments in the form of data in a table in a database called the catalog. - it takes a while to prepare for editing, each time (regardless of library preview availability).
That said, the sliders are available and you can go to work before loading is complete, granted it still takes a second or two, or three, depending on file and machine..
Note: you can turn the "loading..." indicator off, which helps to feel better about editing as soon as Lr is ready (before loading is complete).
PS - because of this, it behooves to do most culling in library module with up-to-date previews, and reserve develop module for true photo-by-photo final edit refinement.
PPS - Make sure you're friends with 'Quick Develop' so you can do initial coarse development work in library module - no loading required...