Lower CPU & system frequency when emulating VGA?

[nxd51]

I tested VGA emulation in MartyPC using the Bochs VGA BIOS. However, I have noticed that when VGA emulation is enabled (instead of CGA), both the CPU and the system frequency decreases (see attached image). Is it possible that VGA emulation requires more CPU performance than CGA and my computer is not fast enough for it? (it's a Haswell Core i5, so quite old.)
I am using MartyPC 0.4.1 on Kubuntu 24.04, I have compiled it myself from source code.
Thanks in advance.

[dbalsom]

Your performance graph looks fine - as long the red line is under '17' (16.7 to be specific) you can hold 60fps.

I think what is going on is that VGA wants to do 70Hz and you don't have a 70Hz display. This probably a pretty common scenario - the way to handle it would be to do frame-skipping, but I haven't implemented that yet.

What I'm not sure about is why your CGA window is showing you only getting 43 fps. Your bus is running at full speed (14.32Mhz) so with the CGA being synchronous with the system it should be emitting 60fps. Puzzling.

EDIT: Yeah, that's definitely what is happening with VGA.

MartyPC queries the virtual adapter to see what refresh rate it wants - VGA will tell it 70Hz and so MartyPC will calculate a timestep based on 70Hz. Those timeslices will be 85% of 60Hz timeslices. It's not just your display refresh rate at play, but your window manager has also locked updates to vsync. If MartyPC actually got 70 ticks it would have emulated enough cycles to get to 14Mhz regardless of your refresh rate.

On windows we usually see a few hundred updates per second so this wouldn't be an issue. In any case, I should be able to detect I'm falling behind and frame-skip as necessary.

[nxd51]

Thank you for the detailed response. My monitor is indeed 60 Hz. I did not know that even CPU emulation is tied to screen refresh rate. I will check how I can disable updates locked to vsync in X11 or KWin.
Regarding CGA FPS drops:
The frequency is usually normal, and my screenshot was unfortunately misleading. I have pressed Print Screen still in BIOS before DOS has booted and it caused a reboot in the emulator. Either this or the launch of the KDE screenshot program was the likely cause of the drop.