On every PC it takes seconds until DO (USB-Version) changes between folders within listers.
Try copying it to your HDD. (Turn on the “use as a dongle” option when exporting it to allow it to be copied off the stick.)
Either the drive or antivirus must be slowing it down since Opus doesn’t do anything different about changing folders when it’s running from a USB stick.
(Assuming the config and scripts on the USB stick are the same as on your normal install, of course.)
On all devices is Defender installed, I use DO portable over years now and actually from a M.2-device (iODD Mini). Most internal disks are SSD/M.2 and since a few months it takes up to 2-3 seconds when switching between folders. In Explorer I don't have this behaviour.
Config wasn't changed (except theming). I don't understand what happened...
Antivirus changes all the time with new definitions and methods of detection. If nothing else changed then it may have.
Explorer presumably is not running off a USB stick.
I will start with Defender and report...