Asus RT-AC66U <-> Synology DS413 Wired speed

[psychedelia]

Hi! New to this forum, and it looks like a pleasant place.

So I just bought a Asus RT-AC66U to connect to my ISP's cable modem (DSL line). I've previously used a TP-LINK TL-WR1043ND and while I think the wired speeds downloading from my NAS has been ok (about 70MB/s), I wasn't truly happy with the wireless speed (around 12MB/s). Now, connecting the AC66U in the same matter, replacing the TP-Link, the wireless speed has gone up (to about 22 MB/S), but to my surprise the wired speed has gone way down - transferring with around 33MB/s. I am basically downloading about 1GB files from my NAS to the Macbook Pro via the ethernet.

This came as a bit of a shock to me as I was expecting it to the very least perform the same as the TP-Link. Does anybody have any good ideas on why this could be?

My setup:

DS413 + Macbook Pro <-lan> AC66U <-wan-> Intelo DSL-router.

The only difference with this setup vs the old is the AC66U replacing the TP-LINK - same cables and configuration on the NAS. No QoS enabled. Both the Mac and the DS have static IPs. Same subnet, etc.
If there is any information I am missing, please let me know and I'll provide it.

Thanks in advance!

Stanley

PS! I'm using stock firmware