I'd love to know more about this.
Do you have a support case opened already?
What are you connecting to? Could you share logs? (here or via private message)
I have the same question.
Recently we upgraded Tableau Server 9.3 to Tableau Server 10 and opening reports based on MySQL Datasource takes x2 load time increase.
Insteat of 1 seconds of full refresh report it's became 5 seconds...
Support case plus logs is probably the best way to go about this.
This alone, unfortunately, does not have any actionable information :/