Yes I have always had this problem it seems to be the nature of DAZ.  This is my way of handeling this and it works very well:  Cut down on the number of renders you do, I did alot of them to make sure I will get what I am looking for.  Each render is kept in memory and after a time it will start re-writeing them starting with the first render and each render keeps everything you have in the scen.  Work in steps and save your scen to disk offten.  Build your scen before adding your Characters.  If possable, pose and dress them  in a new wildow and save.  A lot will depend on how complex your scen is,  the more you have in 1 scem the more your puter must add and DAZ has to hold. One other thing I do when it starts to slowdown I will take a break and shut DAZ down thous clearing all saved renders.  Yes  I spend 8 to 16 hours a day with DAZ, I am always building art and loving it.  Though I spend a lot of time in the program I know little about it and for me it is just what I need, I load, pose , render 80% of my time, the rest of my time is just playing and having a lot of fun creating new characters.