An update: Tableau Support contacted me on this and we manage to figure out what was happening:
1) The issue happened only when visualizing the dashboards embedded in Sharepoint, and via IE10. All good when using Chrome, or when accessing the dash directly from Tableau Server;
2) The problem seems to be around compatibility between IE standards and sharepoint's webpart standards.The latter followed IE8 standards, while users have IE10.
3) For some reason, this incompatibility didn't seem to bother the dashboard performance while on Tableau Server 8.0. But after upgrading to 8.2.2, it became an issue. I still don't know why, and neither Tableau Support does.
Workaround: adjust browser or document standards so they match. To do so, press F12 on IE10. and on the console menu, change "Browser Mode" or "Document Mode" to adjust the standards.
It is a specific use of Tableau, but I hope this can be of help.