i have 300 girls if it matters
Potentially, if by "have 300 girls" you mean you followed some of the modding advice here and made it so you have 300 girls in your brothel, yeah, that can cause issues. There is also, in general, slow down that occurs as you progress through the chapters, unlocking new areas and being able to hold more girls in your brothel, just because each chapter is that much more data the game has to track.
For example, in the first chapter, you only have access to the slums, and a total of 6 slots for girls between the brothel and farm. Putting stores and other special locations aside for a moment, the slum itself has 6 areas to visit, and each of the 6 areas can have a maximum of 3 girls present, for a total of 18 free girls running around maximum. Combine this number with the 6 slots between the brothel and farm, and you get a maximum of 24 girls outside of the slave market that the game has to keep track of, with all the stats included.
Fast forward a few chapters, however, to around chapter 4, and you now have access to 5 areas. Each of those areas has 6 subsections, and each subsection can still hold a maximum of 3 girls, so now you are looking at up to 90 free girls the game has to keep track of, with all relevant stats, personalities, etc. Combine that with the brothel and farm (which can hold 4 times the chapter number and 2* the chapter number respectively), and you get a total of 114 girls the game is keeping track of outside the slave market (I would include that too, but while the number varies it has a set range that doesn't vary by chapter). When you stop to think of everything the game has to keep track of about each girl in your employ, that is a massive chunk of data that the game needs to process in order to keep things running. That also isn't accounting for the additional stores, material sites, mechanics, side stories, etc that also come with the chapter increase either. All in all, the further you go chapter wise, the more slow-down you start to see, even on a brand new machine.
There is no real "fix" for this, because the game needs to keep track of all that data to function, and you can only optimize the process so much. That being said, it sounds like there might be something screwy going on with your install, because this particular problem doesn't really "go away," so a fresh install could help if the slowdown isn't related to the general issue of increasing numbers of variables.