I have also upgraded from T7 to T8 and am experiencing the same problem with one of my Dashboards. This has been a huge waste of time and has led to nothing but dissapointment. At this point, I have no choice but to revert back to T7. I plan on submitting a defect report to Tableau in an attempt to get resolution.
A few things to try:
- Upgrade to 8.0.2 (released Tuesday) if you haven't yet. We've made some improvements since 8.0.0.
- Create a performance recording (Help > Start Performance Recording). That might help you find the problem. It certainly would be helpful to send this recording (save as TWBX please) to support if you do submit a defect report.