I've had the OP's issue, except mine was with graphic images.
For example, I had a set of 30 animation frames that were 800kb's each. It took forever to preview. But when I cut the images down to between 130kb's to 200kb's, the preview loaded much quicker. Of course, I realize that smaller images load faster than larger ones.
However, I know it's not my computer that caused the slow load times prior to the cutdown. My CPU is not the best, but at 2 and a half Ghz, and 3 GB's of RAM, it should handle everything in Construct fine. But there are quite a few hiccups with loading and saving even smaller images and layouts. When I've used the same images in Visual Studio/XNA, they loaded quickly at even large memory sizes.