diff is really slow if it hits binary files due to its small chunk size.
; time diff /bin/gs /bin/gs
0.005u 0.055s 1.800r 	 diff /bin/gs /bin/gs

and with a 32k buffer:

time 6.out /bin/gs /bin/gs
0.001u 0.012s 0.485r 	 6.out /bin/gs /bin/gs

Reference: /n/atom/patch/applied/bindiffspeed
Date: Mon Sep 28 05:54:34 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/diff/diffreg.c	Mon Sep 28 05:53:04 2015
+++ /sys/src/cmd/diff/diffreg.c	Mon Sep 28 05:53:05 2015
@@ -316,7 +316,7 @@
 	flushchanges();
 }
 
-#define BUF 4096
+#define BUF 32768
 static int
 cmp(Biobuf* b1, Biobuf* b2)
 {