Most of the time, when your Order Summary reports and pick lists are not loading very quickly, we are trying to load images for your reports and are likely being unsuccessful, so it's causing the slowdown. Luckily, it's easy to turn off the images for your reports to see if this is the case.
Head to your Account Settings (the gear icon in the upper right).
Next, select Printing and then Printing Setup from the sidebar on the left.
Here, you'll click on Document Options under "Options" for Order Summary Report or Pick Lists.
Just uncheck the box next to "Print product images on Pick Lists" and check the box next to "Don't print images on the Order Summary Report" like you see below.
Now, these two reports should generate much more quickly. If they do, you may want to check with your marketplace(s) to make sure that the images are being sent over correctly and that there are no dead links.
Once you have fixed your image sources, your reports should generate a bit faster than they were previously. If you still have issues with your reports or pick lists not loading quickly, please open a support case so our team can take a look.