With the update to the most recent version of Opera (Version 67.0.3575.137) there is no longer any slowdown in the display.
It is possible that in my case this slowdown was linked to a virus detected on my computer: Koiot.gen. I have not found any description of the effects of this virus.
Another possible cause in my case is a bug experienced by several macOS Catalina users, inducing a kernel panic, which would be linked to the [Intel] GPU (Graphics Processing Unit): Constant Kernel Panics – userspace watchdog timeout: no successful checkins from com.apple.WindowServer.
