I have a 2 gig SD card and a couple of 8 gig SD cards but the 8 gigs (one is class 6) are slow and take about 30 - 40 seconds or more to load up / navigate through.

The 2 gig is quick enough so its either the size of the card or the amount of stuff on the card.

What is slowing it down, the card size or amount of stuff on the 8 gig cards?

Can you lay hands on the specifications for your various cards?

As you suggest, it's worth looking at what speed they're rated at:

Secure Digital card - Wikipedia, the free encyclopedia

As suggested, its the speed of the card thats the issue. You may have a class 6 card, but not all class 6 cards were created equal. As you can see from the link posted by Willum class 6 has a variety of speeds from slow, to well very quick.