Did this problem only appear in Excel? How about other programs?
To narrow down this issue, Please do the following:
- Use Ping command to test the connection between the test client and the server when problematic users feel slow. Please record the response time and let us know.
- Temporarily disable all the redirections (Driver, Printers, video, Clipboard, etc.) & experience (Font smoothing, Desktop background, Visual styles, etc.) from terminal services client and then test the issue again
- Temporarily disable all the anti-virus software or firewall between the test client and the server. Check the issue again after that.