RDP Sessions are slow ...what does the affect of Color Depth have on the user experience?

We have an array of terminal servers to which our customers RDP and launch a Windows Forms application. We get a lot of complaints that the sessions are slow. So, when one our technicians shadows the customer session, the customer says, 'wow, everything is responsive now.'

Our data store is MySql 5.5, servers are 2008 R2.

Edit: additional information
Each customer has their own MySql database from which to work
Terminal servers have 25Gb RAM
Most customers have business class internet connections
Server upload via 1 DS3 and 1 100Mb fiber

What are common reasons - network settings, architecture - for slowness over RDP? We are trying to figure out how come, after shadowing, the customer's experience improves so much.

Edit @syneticon-dj mentioned ColorDepth in his comment - now I can focus my question...
As far as I know, we allow our customers to set their RDP color depth to their liking. Is there a throttling mechanism we can look at on the server side? And what would be a minimum setting for a Windows Forms application?

RDP slowness could be due to one of the following: - poor internet connection (you or the customer may need a higher upload speed) - insufficient RAM on terminal servers - contention for mysql resources/objects