Ubuntu 18.04 slow after 2019 update

Answering my own question after 3 weeks of despaired search and a fruitless complete 18.04 reinstall including all packages I had before. Finally diagnosed and repaired by an expert family member inspired by question 1021748.

The ondemand CPU governor switched to "powersave" mode after booting up. This lowered the CPU frequency to even below the minimum set frequency of 400MHz, and stayed there hovering around 380Mhz (inspected via lscpu).

This happened when the governor switched the CPU to powersave mode:

cat /sys/devices/system/cpu/cpufreq/policy*/scaling_governor

Switching the CPU governor to performance mode solved the problem (for policy0-3):

 echo performance | sudo tee /sys/devices/system/cpu/cpufreq/policy0/scaling_governor

Permanently removing the ondemand governor, made this survive reboot, keeping the governor set to performance:

systemctl disable ondemand

No idea why the powersave mode turns out so bad and what went wrong where in the April 26 18.04 update and is still present in the reinstalled version. Some posts hint that the ondemand governor doesn't play well with the Intel pstate scaling driver... Anyhow, my question wrongly asked about gnome (I took it out). It was the CPU control affecting everything.