This issue is most noticeable when saving records or switching tabs in a record as response times are significantly degraded from previous versions as you may notice a very large increase in your Waiting (TTFB) when comparing performance via network trace in a web browser.
This issue is caused by some new settings missing in the web.config file. Starting in the above versions of Archer, some additional settings have been added. If the settings do not exist in your web.config file perhaps something failed when the installer tried to update it or an old file was copied in place after the upgrade.
In order to confirm you are experiencing this issue look for the section below in your Archer Web.config:
Then save changes to the file and perform an IIS reset once the web.config modifications are complete.
Alternatively, running the Archer installer with the correct permissions on the account running the install. (Run as a different user if needed and ensure Full Control over the file/folder for that account).