If have many HaldCluts Rawtherapee is loading more slowly. And when to choose shot for editing, Editor is start on slowly to and this effect repeat when I want work with Color section. At a choice Color section system to use one of four processors (only in this time with this operation). If to delete or to have some numbers HaldCluts - Rawtherapee start very fast and work without problem. May be You know what the probleb. System: 4 cores processor, 8 Gb RAM, SSD (not full), Windows 7.
(English is not my native language, sorry for mistakes 
This is because RT reads the HaldCLUT folder on start up. If you have a massive number of cluts, reduce the number to regain the speed.
And maybe place those HaldCluts in a good SSD hard drive connected to a SATA3 port (or even a NVMe port if you have it)?