How can I optimize NAT on my home network?

Installing and Using OpenWrt Network and Wireless Configuration
1

I have one OpenWrt TP-Link Archer C6 v2 device set up as my main router, behind a proprietary modem/router in bridge-mode. Next to that I have two OpenWrt TP-Link Archer C6 v2 devices set up as separate access points in my home.

I made a mistake in setting up OpenWrt and for a while, I was in a IPv6-only situation. Many websites were hard to reach or didn't function properly but I did notice that connecting to websites was very, very fast.

I did some research on it and my best guess was that this was due to skipping NAT.

The IPv6-only situation is resolved now but right now I would like to know if there are ways to optimize NAT on my IPv4 connection without sacrificing protection.

2

No, IPv4 needs NAT to translate the public IPv4 address in local private addresses.

But you only need NAT once, as the ISP modem is in bridge mode your OWRT router will have the Public IPv4 address and needs to NAT.
If you have multiple routers then set those up as Dumb AP's