Ok, so you guys are saying that this thread is not accurate and I should disregard it? (ie. The slowdown I'm seeing has nothing to do with Hardware NAT support not being implemented in the OpenWrt firmware)
In that video he says that the wifi speed maxing out at around 345Mbps is related to CPU power, as was pointed out earlier in this thread.
It seems like @mk24's answer is still the most representative of this, even though he doesn't specify the actual reason why this is a CPU issue other than OpenWrt is "general purpose firmware which will require more CPU power for the same speed". Sounds a bit "hand-wavey" and unclear to me. Why wouldn't there be some efficiency optimizations by the OpenWrt developers for the Archer C7 to improve this CPU issue? There should be a specific reason, which is what I've been trying to determine, but it sounds like nobody can or will share it beyond what I quoted above.
Which means I still have no idea why OpenWrt is slower than the stock TP-Link firmware for the C7 and neither does anybody in this thread (again, no offense to anybody, just trying to find a solution here). The fact is that with the TP-Link firmware my connection was faster and then it slowed way down with OpenWrt. Other posts on the internet confirm similar experiences. I still couldn't tell you why this is the case other than it is a "CPU power issue". But, that very same "CPU power" powers the stock firmware just fine. Clear as mud. 
Okay. Well, this has allowed me to learn a lot about routers and firmware, so thanks for that, but there is no conclusive explanation as to why OpenWrt is slower than the stock firmware.
All I really want to do is get a firewall and some sort of tracker-blocker set up on the network like Pi-hole or Adguard and I think @anon82289434's post might be the best way to do this, after all. Thank you again for that.
A few questions about it, though:
- Is Adguard able to block direct IP address connections?
- In the setup you described, does that effectively make the Archer be upstream from the Pi or a sort of peer?
- Is there a good firewall available for OpenWrt that has a GUI in any way similar to Pi-hole? (or, does Adguard provide this?) Does it list all outbound connections including attempted outbound direct IP address connections?
- The guy in that video says the Archer's built-in Wifi hardware could 'easily do more' than the observed 345Mbps. That is very encouraging. This suggests that your setup could provide higher speeds than this, by the sounds of it?
- How would I install OpenWrt on the Pi? Is it easier than I think?
Thanks again guys and I appreciate your patience. 