I installed a Windows universal forwarder on version 5.* 2 years ago, and upgraded it to 6.1.* recently.
Comparing the old upgraded UF versus a fresh install, I noticed that the old one was using much more resources (cpu/memory) to collect the Windows logs (perfmon, wineventlogs ...)
It seems that the only difference is the version of the Splunk Add-on for Microsoft Windows that was installed by the MSI installer.
I am not using a deployment server. Does it mean that the upgrade of Splunk does not upgrade the Windows TA?
That is correct, upgrade does not install the latest version of SplunkTAwindows, only new installs do. I have seen cases like yours and upgrading SplunkTA to the latest version has resolve issues with High CPU/Mem. To find out what version you are on go to the etc/apps/splunkTA_Windows/default/app.conf and look at the build.