With mgba freshly opened, without executing any game, the CPU usage can increase a lot when showing a lot of texts in menus (for example, the preferences screen or listing all roms in a directory. Are you using a graphic filter to soften the text?

This behavior only affects a single core, it doesn't really impact the usability of the emulator (the interface responds perfectly) and the CPU usage returns to normal as soon you open a rom (closing the interface), so it doesn't impact the emulation. So it's very low-priority.

The CPU usage was measured with Status Monitor v0.6.3 overlay app. There's also the possibility this resources usage applet is buggy and giving wrong values and not mgba.

Some screenshots of the CPU usage, higher the more text mgba menu renders.

69% usage, quite lot of text

22% usage, little text

99% usage, a lot of text

Even while emulating Crystal the CPU usage stays at a low 11.8%

...but if you pause the emulation and open the preference option, it maxes out.