The same problem to me with: Lazarus 1.8.2 QT5 VERSION + (Ubuntu Mate 18.04 64 bit OR Linux Mint Mate 14 64 bit)
4 cores, one of them is at 100%, working with Lazarus IDE or running the compiled program alone!
The same executable file, moved to a Manjaro 18 64 bit, is working fine, 0.1% CPU only.
I uninstalled the Lazarus QT5 version and replace it with GTK2 version then clean and rebuild the project.
Tadaaaaa, the GTK2 is not eating 100% CPU anymore on Ubuntu Mate 18.04 !
Any ideas?
Thanks
Teo