How do i set up the GOG DOSBox Configurator for the best performance ? Any help will be appreciated .

That's very much a matter of opinion... by default it will go for 2x native resolution with doubled pixels, in other words 640x400.

For me, I like using a better scaler (even if its use is limited in Rayman because the look isn't that cartoony), so I opened up dosboxRayman.conf and set scaler=advinterp3x . That had the drawback for me that it didn't get full-screen, so I added a custom 960x600 resolution in my display driver's settings and set fullresolution=960x600

Also make sure to set the output method to one of the accelerated ones to make use of graphics acceleration for scaling to fullscreen:
output=ddraw or output=opengl