Does any of you use NFS or SSHFS on the router's USB HDD? How feasible is this?

Is it feasible anyway? I am trying to do this for some light gaming. I don't have much space left on my laptop, so gaming via NFS or SSHFS it is. It works well enough for my NAS, but what about my router? I don't wanna use my NAS just for some gaming, if the router is enough for it. What about the power it will draw? I am using Asus RT-N18U.
A side note, while SFTP'ing, the max speed I reached was 5MB/s. Although with torrent downloading it gets over 9 MB/s. Can it be faster than this? What are your speeds while transferring some files between the USB HDD and your PC?

A router is not NAS, if you want performance, you're not going to find it in using the router as such.

sftp/ sshfs being slower than directly writing to the disk (torrent) is not unexpected, as that involves the cryptographic overhead as well. In comparison to modern NAS hardware, 1*800 MHz ARM cortex A9 is not exactly in the running, nor are 256 MB RAM and the disastrous WLAN hardware making this bad situation an better.

From a security point of view, you generally would want to reduce the attack surface of your router, not broaden it by running a file server on top of it, it does make sense to keep these roles separate.

It depends on the CPU as well, your RT-N18U is very old and the CPU won't give you much speed, SFTP has encryption which makes it even worse.

Previously I tested with Asus RT-AX82U (not OpenWrt compatible) and the USB transfer speed can hit gigabit without problem due to a faster CPU.