I initially noticed that PDF’s were becoming increasingly slow to load when using Thunderbird. Once opened, usually after 30 seconds or more they then loaded quickly as expected.
Turns out this is an issue with Adobe Reader and the fix is to disable the sandbox protection under enhanced security.
In Adobe Reader go to Edit> Preferences > Security (enhanced) and deselect “Enable protected mode at start up” and click OK. (very important to click ok)
