I think I’ve inadvertently found the cause of the problem. When working on another job, having been given a project from the same client, the same issue was occurring but slightly different. The project opened but not as fast as it should. There were a couple of comps that have only text in there and these comps would hang and be unworkable.
Turns out, it was a problem with the font. I deleted the font that was being used and reinstalled it and voila. Cooking on gas now.
Thanks for your feedback guys.