I have spent a number of years trying to get Asus routers running a full featured media server. Great routers, great software (when you add in Merlins enhancements and Entware-ng).
But, not the best platform for media servers or other "non network infrastructure" applications.
The Linux kernel is going on 6 years old (2.6.36) and the memory footprint is fairly light. Fine for routing, switching and light services. But, when you try to add functions like a DLNA server for large amounts of media - it struggles.
What I ended up doing was adding a co-processor to my RT-ACXX router!
I had originally started with a Raspberry Pi. I am now using a Banana Pi. These little boards cost around $40. Add a case for approx $8, a USB power supply and an SD card. Other than the case, I had a few SD cards and USB power supplies in drawers!
I have moved over to the Banana Pi due to the fact it has 1 GB of DDR3 DRAM and a 1Gb Ethernet. The Banana Pi also has an external SATA connector and a dual core 1Ghz ARMv7 core, the Raspberry Pi a quad core 900Mhz Armv7 core with a 10/100 Ethernet port.
Both run Raspbian (a Debian tuned for the Pi's and Linux kernels in the 3.4 and higher vintage).
https://www.raspbian.org/
I plug the Pi into the AC-RTXX router and give it a static address. Now, I can add all kinds of applications, services etc. There are more then 35,000 software packages available.
I am running the Plex Media Server on the Banana Pi. Works well. Very easy to install:
http://www.htpcguides.com/install-plex-media-server-on-banana-pi-with-bananian/
Like I said, love the RT-ACXX products, but for more heavy duty applications, I believe a "Pi" for roughly $50 is a great compliment to Asuswrt-merlin.