Selectively slow Ubuntu

[mahesh]

After 6 years I had to discard my Ubuntu 16.04 and made a fresh installation of Ubuntu 20.04.
To my surprise some programs are painfully slow.
I noticed that my favorite browser Opera didn't behave normally. It can take up to 10 seconds to enter a single character in the search field.
The same applies to Thunderbird.
Other programs like Firefox, VMware player and Libre Office are snappy and behaves normally.
Programs run in the terminal like Maxima behaves at full speed.
To get an idea of what is going on I installed Vivaldi, Konqueror and MS Edge. They all behave in the same dreadful way.
The only browser I have tested that behaves normally is Firefox.
I can live with Firefox but somehow I must fix Thunderbird.
Can anyone please tell me what is going on?
H