Thank you for the scene and timing reports.
I'm glad that the main issue of slow rendering is fixed, but also puzzled like you as to where it came from exactly ..
I recreated a heavy scene with animation based on you scene and settings, but couldn't reproduce the issue..
It's likely that some deprecated settings are now automatically turned off when we open the Preferences or perform some action; I'll try to investigate more and add a message / warning if that's the case..
For installing older versions of the Octane plugin, without the installer, I'm not sure it's possible without some Windows permissions.. but you can try:
Basically, you just need to copy 3 files to the 3dsMax folder. (while 3dsmax is not running..)
That's where Windows might forbid you access.. or just ask you to confirm the copy..
To check that first, see if you can copy a dummy text file, to C:\Program Files\Autodesk\3ds Max 2020\ (or the location of your actual 3dsmax)
Note that in Octane preferences, the Tools menu has 'Explore 3dsMax folder', that should open this folder for you.
Using 3ds Max 2020 as an example, the 3 files that you will need are:
C:\Program Files\Autodesk\3ds Max 2020\octane.dat
C:\Program Files\Autodesk\3ds Max 2020\octane.dll
C:\Program Files\Autodesk\3ds Max 2020\Plugins\Octane3dsmax.dlr
You can find them in previous versions's installer file
Eg, if you unzip OctaneRender_Enterprise_for_3ds_Max_2020.2.1_-_11.06.exe, you will find
octane.dat and octane.dll in the root folder, and several Octane3dsmax.dlr inside a folder for each of the 3dsmax version
folders '$_3_plugins' and '$_4_plugins' for '3dsMax 2013' and '3dsMax 2013 Design'
$_19_plugins and $_20_plugins for 3dsMax2021
(and $_21, 22.. for latest max2022..)
Not fully sure that this could help, but a tool like elevate.exe can run a batch file with administrator rights, so that could be a way to copy those files..
Also another idea, could be to install 3dsMax to a non standard location, (eg C:\myMax\Max2020), that you could copy files to without access problem..
I hope that helps