cmake – Compiler issues using Raspberry Pi 4B

I am a newbie at ROS, so there is a lot of things to learn.
I successfully installed Ubuntu Server and ROS2 Humble on my Raspberry Pi 4B (2GB).
When trying to build the examples with colcon, the system after some time gets extremely slow having almost no reaction anymore.

I found out, that the root cause is not enough memory when 4 compiler runs are executed in parallel. I tried to enforce only one compiler instance by

export CMAKE_BUILD_PARALLEL_LEVEL=1

but this did not help. There is still 4 compiler instances running at the same time.

Can anyone explain me how to block parallel compiler runs?

Thanks!