Cesium Billboards Load Time

I have a question related to performance of adding billboard entities. I realized that adding for example 1400 billboard entity with same image is faster than adding 1400 billboard entity with each one having different image. There is a system part in performance monitor that does something while loading billboards. (i think it also depends on computer specs and how chrome handles these system tasks which i really don’t know) After some time, application speeds up and we lose less frames. Do you know the reasons of this load time? Is it related to texture atlas and gpu? What can i do to maybe lower this load time? Do you recommend adding these billboards partially? I am also using suspend and resume events while adding these billboards with request render mode. If you can give me some ideas I will be so happy. Thank you so much.