I tried to make a little waterfall. I attached the hip file. If I want to render it from for example frame 55 to frame 60 the 55th frame rendered in about 30 minutes, the 56th is in about 1 hour the 57th is in about 2 hours and so on. Isn't it normal right. I have an I7 6700K with 32 GB RAM and an Asus GTX1060.

It could be you are continuously adding more particles into the simulation. Try adding a sink to the bottom of your waterfall or somewhere “off camera” to try and keep your particle count consistent from frame-to-frame.

Thank you.