Both are quite weak for today's standards. Furthermore you wouldn't want the file transfers to choke your internet connection. Either separate the functions of router and nas, or get a beefy router with multiple cores and over 1GHz freq.
I have tried most common formats.. fat32, ext2,3,4 before finally going with NTFS. Fat32 has the same speed as NTFS as i observed. Ext2,3 seemed a bit slower. Ext4 speeds are quite close to NTFS and fat32.
Not using fat32 because of its 4gb limit.
Not using ext4 due to not able to mount the partition in windows pc for file transfer.
The ASUS RT-N13U B1 is based on a ramips RT3052 SOC, single-core, 384 MHz mips 24Kc, it's severely underpowered (CPU bound) for your plans - and the figures you get are in line of what I'd expect for this device. Yes, an archer a7 will be faster than that, but it won't be good either (less than twice the performance you're getting now). These routers haven't been made with NAS uses in mind and aren't capable of doing with satisfactory performance, modern multi-core ARM routers tend to be better in this regard, but even those aren't made for that task either.
trendy's advice to look into alternatives, e.g. RPi4, is the only sensible choice here.
I have a PI4 with Ubuntu 20.04 LTS on it. Used for docker containers and a Plex server which uses an external 4tb off a powered usb hub. Recently upgraded from using a usb3 stick as its root fs to a external SSD (thus the need for powered usb hub as two powered usb3 devices is too much for the pi to supply power to)
Works beautifully. The ssd is FAR better for read/writes but running it off a usb3 stick is still acceptable. It worked ok from the Sdcard but sdcards are prone to corruption if the power glitches.
How much performance increase are we talking about here with exfat in kernel directly compared to ntfs? Can you be able to give me a general idea?
If it's more than significant then i would definitely try fourth RC.
There was a time when ntfs seemed heavy on system, it was taking all of cpu when downloading something off of aria2, but after playing with mount options a bit, managed to get it going again with low cpu utilization. It didn't improve file transfer speeds though.
I'm using a Pi4 with ubuntu 20.04 not openwrt however my external drive is more than fast enough for streaming my Plex server
root@ubuntu:~# hdparm -tT /dev/sdb
Timing cached reads: 1852 MB in 2.00 seconds = 927.15 MB/sec
Timing buffered disk reads: 162 MB in 3.00 seconds = 53.94 MB/sec
for comparison my SSD via adaptor is :
Timing cached reads: 1844 MB in 2.00 seconds = 923.04 MB/sec
Timing buffered disk reads: 956 MB in 3.00 seconds = 318.21 MB/sec
:edit: also exFat is perfect for swapping between windows or linux. I however cheat and just SMB mounted the external for lazy copying over the network. However when i do have a fair bit of data to throw at it, I power down the pi and plug hdd into my pc for direct usb3 transfers.