We've seen some slower performance specifically around particular CPUs with missing instruction sets (resolved in 10.5.2 - Tableau Server 10.5.2 | Tableau Software ) or workbooks that use complex calculations that need to be optimized in Tableau Desktop and republished. https://onlinehelp.tableau.com/current/pro/desktop/en-us/help.html#extracting_optimize.html
Both of these issues were edge cases, with less than .5% of customers/workbooks affected, so to have them both affect your environment is very unlikely.
I'd recommend opening a case with Technical Support so they can help root out the cause of the slowness in your environment.
Last Monday our 10.5.1 production environment, which had been running fine for several weeks, started experiencing very slow performance, even timing people out of sessions. When we looked at the normal places (CPU use, memory, disks not filled up etc) and had our telecomm guys check for issues with the network, we discovered that a few Microsoft OS patches had been applied to the servers the prior day (Sunday). One was kb4088879 which addresses a vulnerability called SPECTRE. I did some research and it turns out this patch tends to cause big time performance issues on Windows servers. We rolled the patch back off Tuesday evening and all was good again. Make sure this patch is not installed on your servers!