I've just rebuilded lazarus with Qt5 (Fedora 28 64bit) and CPU usage is normal.
Are you using -dQtNativeDialogs ? IMO that's related to gtk theme which is used by Qt5 on gtk based desktops (gnome, metacity etc) and I've spotted higher cpu usage with such theme.
Recent lazarus trunk does not use native Qt5 loop but LCL one, so you can test your app against current trunk eg Lazarus 2.1.0 r59308 FPC 3.0.4 x86_64-linux-qt