As one of my games (Master of Magic) is running quite slowly in fullscreen mode and changing the graphics settings with the "Graphic Mode setup" doesn't solve that problem, I would like to know where I can change the dosbox configuration. When I try to access it in the game folder it says "run dosbox configurator from the start menu". But I can't find it there (only an older version of Dosbox that I previously installed and where changes don't influence my gog-games).

So how and where can I access the dosbox configurator?

Oh, if that helps solve my problem in any other way, my specs are:
Sony Vaio
Intel Core i5-2430M CPU (2.4 GHz - 4 CPUs)
8 GB RAM
AMD Radeon HD 6470M

PS: I just realized the game is generally too slow, not just in fullscreen

Keep hitting Ctrl-F12 until the speed is correct.

Alternatively you can edit the Dosbox%.conf file with notepad and set cycles=max, which works for most games.

CTRL-F11 or CTRL-F12 usually does the trick for me with most DOSBOX games, including the Alone in the Dark series.

If you want to fine-tune the configuration, launch the game using any existing DosBox frontend, e. g. D-Fend Reloaded or DBGL. They all are compatible with GOG releases.

Haven't heard of either one of those before, thanks for the tip!

Should be in Start/GOG.com/<game name>/Graphic Mode Setup

That's only for a few limited graphics options. the .conf file is what's needed for real tinkering.

I know, but that's what it seemed like he was asking how to find.