(sorry about my english

My problem:
When i open a picture this is going very quick! But when i want see the next picture in this folder i have to wait 1min. After this thinking-minute the changes goes also quick an without problems... Why the program need this minute... I had a lot of pictures(40000) but they are all in other folders... So the program not have to load all this pictures before it can show me the next...
I hope u understand my problem/question?
Thanks for ur help!