Eclipse Keyboard Lag

I was using Eclipse 2020.03 on Kubuntu Linux, with of course GTK dark theme (if this detail matters) to be homogenous with my Plasma desktop Dark Breeze theme.
Editing a Java application was just a mess.
There was an abnormal keyboard lag while typing, and in particular the content assistant was not inserting selections if I was hitting any char on the keyboard just after the Enter in order to select the choice.
The following shows how things were: note the delay between the disappearing of the content assistant (because I selected the proposal) and the appearing of the selected element as text.



How to solve?
I tried to set an higher activation time for the content assistant, and at first it seemed to work, but after a while the above behavior was resumed. I therefore increased the base memory of my Eclipse in eclipse.ini setting:



and restarted Eclipse.
So far it seems to work!