Well, overclocking is in my opinion no solution, because maybe you'll achieve 10% more, thats it! It isn't worth the risk to brick your router.
I would suggest to use your Asus as AP only, without any routing function (an other router can handle this). Further, I wouldn't start everything when booting - just when needed. Then I would throw out any unneeded modules in order to get more free ram, use lsmod - usually audio modules are never used as well as others (dependent to your config), but be careful some modules will show you unused and it's not true. In any case when you get more free memory, you router will work much better (swaping for instance takes a lot of cpu power)
When everthing must run on just one device, you can try to set priority of running softwaare to different levels.
For instance: give samba the highest priority:
echo "Starting smbd:"
/opt/bin/nice -n-20 /opt/sbin/smbd -D;
echo "Starting nmbd:"
/opt/bin/nice -n-19 /opt/sbin/nmbd -D;
Please read: "man nice"
Priority settings are -20 (highest) up to 19 (lowest), default=0
Have fun