Seems to be a network/server issue. If you start up something like Firebug and activate "Net" tab you can see how images are coming in, sloooowww. Once loaded there are no problems. Your slideshow is actualy way too fast for my eyes :)
How to fix? Faster host and/or stop using images up to near 4000x3000! Some are gigantic. I am not too much in to images but if you look at size they seem pretty optimized but when so large they still can be way over 1mb. If you resized them to be max 1200px wide or something, while not messing up quality!, that will definitely help.
Also you have constantly 2 files not found, 404 errors. Never a good thing. See them here http://imm.io/PxaF or better http://www.webpagetest.org/result/121218_9Z_KE8/1/details/ 1 from drop down menu plugin I dont know where the other comes from. Is not even a file.
jQuery Colorbox seem to handle monster images ok but may be it will also run a little faster with less to squash. Less cpu usage perhaps. From checking network load times it has nothing to do with slooooowww issue. As everyone else jQuery Colorbox is waiting.
If hooked on speed you might want to put an "expire header" on those big images. That is not something to mess up with or rush in to but if you check HTML5Boilerplate .htaccess file, which set these things up https://github.com/h5bp/html5-boilerplate/blob/master/.htaccess and scroll down to expires header you get the idea. When my browser HAVE downloaded a JPG KEEP it for X number of days/weeks/months. Do not request another download when revisiting. Works best for static stuff, like your images. Issues: If you change image but not file attributes I never get to see your changes, not until I empty cache in browser and force a redownload. This is why they say about "expires headers"
They assume you control versioning with filename-based cache busting
Assuming :) You need full understanding for this before doing much, no copy and paste until then!, but is a very good idea for image heavy site like yours. Much to gain if done right.
Make no sense because it used to be fast? Hence use of word "suddenly"! Well may be I am wrong but servers/hosts can go up and down in speed. Good weather/bad weather.