Looking for an option/workaround to limit the diff content (shown in the bottom panel) for any particular file (e.g. to first N lines only), as the patch/diff may be too big and causes the GUI to freeze. I did try the different options accepted by git diff, but don't see any way to pass them to gitk (unlike git-gui). Note that limiting the number of lines of context does not work since the diff will still be shown with zero context.
You will have to change gitk's source code. It's one big Tcl/Tk file you can edit directly or make a copy of. It's location may depend on your operating system. If you are on linux or macos it's probably /usr/bin/gitk or /usr/local/bin/gitk.
Inside the procedure diffcmd, change this line:
set cmd [concat | git diff-tree -r $flags $ids]
You could add diff-tree arguments, or simply pipe the diff output to another program, like head:
set cmd [concat | git diff-tree -r $flags $ids | head -n500]
This applies to the complete diff, not just to one file.