Tableau Server is probably caching the view for quicker retrieval which is why it is loading so quickly. Tableau Reader utilizes the .twbx and needs to load everything every time you open the document which is why it is slower (It does have a limited cache but not to the scope that server does). It is also a good resource to go through the Optimize Workbook Performance that Tableau has: Optimize Workbook Performance - Tableau
It would also be good to check to see if the workbook you are opening in reader is on a Local Drive and not a network for the best performance.
In regards to the 200 dpi, are you going to have people printing it or is it just for computer use? If you are only using the computer to display it you might be able to reduce it to around 72 (not lower or it will be grainy) to see if that helps.
One last though would be to double check that reader and desktop are on the same version.
Thanks for the help. I think most of the performance tips relate to the dashboards or file as a whole, and this issue only is an issue for that particular dashboard that has a lot of images and text on it. In Tableau Desktop, the time is about 5 seconds, whereas in Tableau Viewer, the time is about 60 seconds. Quite a difference. Both are on network drives.
How confident are you that changing from 200 dpi to 72 dpi would make a difference? I'd rather leave it higher in case they want to print the dashboard (it's a help section).
I can't put this on a local drive - it needs to be accessible to all staff.
What I'm guessing I'll do is just create a button with a link to an external help file, since Tableau evidently isn't good at optimizing images and text.