Well Netgear sucks in firmware department. But Asus products (and I have deployed MANY) are very stable and the updates are frequent.
Yes there is not performance impact since all of it is hardware accelerated. That is a fact. You can look at Tims CTF scores in his review of a router like RT-AC86U.
And here Synology explains how the made a IPS engine that handles gigabit speeds:
https://blog.synology.com/building-an-intrusion-prevention-system-for-small-businesses-and-homes/
So yes almost no performance impact. I have 1 gbp/s WAN and a GT-AC5300 with all bells and whistles active (adaptive Qos, IPS, etc). It handles 1 gbit fine. So what you are saying is simply not true and it does not help your cause that you don't know this or question it.
Yes privacy is problematic with the AiProtection feature. But I live in a part of the world where we have governments that actually care about privacy so they have to adhere to GDPR. Besides it is the same thing as running any antivirus program on your PC...or even just Windows it seems lately. So it is a none-issue in that context.
PS: I have had the USG, cloud key (1st gen), Ubnt PoE Swith and three AP Pros. I do have first hand experience with their products.