If you want to disable Alt+<N> shortcuts that switch between tabs, do not comment out or delete the corresponding entries in the “accels.scm” file. Just replace the field values with blanks: “”.

Xfce Terminal is based on the Vte terminal widget library, just like gnome-terminal. Vte is probably not the fastest terminal emulation library on earth, but it's one of the best when it comes to Unicode support, and not to forget, it's actively developed. That said, performance is still an important issue for a terminal emulator and Vte with font-antialiasing enabled can be very slow even on decent systems. Xfce Terminal therefore offers a possibility to explicitly disable anti-aliasing for the terminal font. If you are experiencing problems with the terminal rendering speed, you might want to disable the anti-aliasing for the terminal font.