I think I've fixed the issue, the feature will be added to 0.205. This is the final behaviour:
- The frame delay slider is only saved if you change it from its 'default' value.
- By 'default' value, GM takes the one from the .ini files (remind the .ini priority hierarchy)
- E.g., if frame_delay=1 in mame.ini, the slider will show "1", and won't be saved unless you change it.
- If you change the default value, it'll be stored in the game's cfg.
- The slider will always have higher priority than command line or any .ini. It's done it this way to be coherent with the behaviour of the rest of sliders that do save currently in baseline (brightness, gamma, etc.).