I was just wondering if anyone was noticing lag while NEI / JEI has the first page of Ancient Warfare open. If so, does anyone have any way to stop it from happening?

What kind of lag are you seeing?
The page taking longer to load? Have you got vehicles on that page?
Or is it a constant lag while the page is open?

As long as the page is open, it's just lag. As in, slower everything (tooltips, moving items, etc.). Every vehicle is on that page at 1080p it seems: https://i.imgur.com/3mYGOVn.png (GUI Scale is Normal.)

I can't see any lag at all so even when viewing the vehicles, but maybe I have too beefy machine and you don't have one like that and that would be the cause.
The only other explanation I have is taht you have some kind of mod that's trying to do something when entities are rendered and that code struggles with all the vehicles that are displayed there.

This is my current system: https://www.bestbuy.com/site/asus-tuf-gaming-fx705gm-17-3-laptop-intel-core-i7-16gb-memory-nvidia-geforce-gtx-1060-512gb-solid-state-drive-black/6293624.p?skuId=6293624

My previous system also had this problem, with an AMD R9 390X, 32GB of DDR3 RAM, 256 GB SSD (Only drive), AMD FX-8350 Processor. All of the mods I use are in this modpack: https://minecraft.curseforge.com/projects/akatsuki?gameCategorySlug=modpacks&projectID=293316

Really appears to be a problem of vehicle rendering lag. I will look into and either optimize or replace with texture icons instead.