If the website is julsfoto .com:
Looking at your start page, you have 4 images in the slideshow at massive file-sizes, ranging between 4 and 13 MB per image. That's 20-30MB just to load images into the slideshow. Why are you not resizing images on upload?
This image
http://julsfoto.com/content/1.index/WoodNymph.jpg is more than 11000px!
Next up ... check your gallery page
http://julsfoto.com/Birds/caribean/. The originals for each image are also massive, for example
http://julsfoto.com/content/2.Birds/2.c ... _287-2.jpg (3000px). Basically, when you access that page, you are asking your server "hey please resize dozens of massive images simultaneously". Even your desktop would struggle with that.
The only reason you don't get an error in the first place, is because you have a private virtual server perhaps, with a high amount of memory available. Just because it has a lot of memory does not change the fact that processing all these massive images will take a lot of time, and requires a lot of processing.
Ultimately, this has a simple explanation: You have a large amount of incredibly large image sizes, and your server will struggle to resize all these images at anything else than slow speed. Sporadically, you may also get "timeouts" or even errors, if the total sum of used memory exceeds allowed amount.
Once images get cached in required sizes, speed will slowly increase. You should still reconsider your uploaded image sizes though ... No users wish to download 10MB sized image, which are scaled down in browser anyway.