Should progress bars accurately reflect how close the operation is to completion during long operations?

I'm creating a simple application. There's a certain operation that will take a long time to complete (~15 minutes), and I'm hoping to make a progress bar showing how close the operation is to completion.

I've notices that progress bars on some applications load slightly quicker at the start than at the end. It might just be because of the operation being performed, but it also seems like a good strategy for keeping the end user less frustrated because it'll appear to be loading fast, but doesn't go quite as fast at the end.

Is this a good strategy for keeping users happier during long operations? What are the pros and cons?