Asus RT-AC88U causes loss of approx: 40% regardless

MERGED QUESTION
Question from neonphelps : "Asus RT-AC88U causes loss of approx: 40% regardless"

Anyone else having this problem? or configuration ides?
Asus RT-AC88U Router
Problem: Speed Problem: 40% loss going thru Router from direct connection to cable modem
Tested on 20Mb/s Brighthouse internet and 100Mbp/s Spectrum Internet
No matter the how the router is configured, router’s firmware version, device used to connect to the router, configuration of the NICs connecting to the router, internet speed, internet provider (they say spectrum’s & brighthouse’s are different networks at this time): I consistently lose about 40% bandwidth when going through the router vs connecting directly to the cable modem.
Only constants are the cable modem and the different internet speed testing software.
New RT-AC88U (about 1 month old) – I do not know if problem has always existed, didn’t pay attention until I decided I wanted to video stream
Direct connection of any PC/laptop to Cable Box results in approximately: 116Mbps
Through Router: approximately: 65%
Tested using AT&T, Verizon & Spectrum Speed tests, about the same results from all
100Mbps+ without router (direct to cable modem)
65 Mbps connection through router
Router directly connect to cable modem (not ether switch)
About same % loss before upgrading from 20Mbps Brighthouse Internet to 100Mbps Spectrum
Approx. Same % loss using:
AC88U with out of the box Configuration & Firmware (380.66.4) (reset the router after discovering situation to see if this corrected it).
Custom configuration & updated to latest Firmware (3.0.0.4.380.7627)
With Latest version of ASUSWRT
Any one of 3 x PCs and Laptops connected (only one connection at a time)
Any one of 2 NICs in a single PC (only one enables at a time)
With WiFi in all PCs and Router turned off – disabled on PCs/Laptops
With WiFi one PCs at a time and Router turned On
Using DHCP to assign IPs & DNS
Hard coding IP addresses & DNS addresses
Hardcoding IP Addr & using DHCP for DNS
Different DNS server addresses
Approx. Same % loss using:
VPN being run or Not Run:
Different VPN providers (most tests conducted without VPN)
VPN being initiated on the PC or VPN being implemented through OpenVPN on the router instead (router configured with latest version ASUSWRT)
VPN slows down internet as expected, but still approx.. same % loss using when running VPN directly on the PC/Laptop and connecting it directly to the cable modem vs running it on the PC/Laptop and connecting it to the router.
I saw that someone had an RT-AC66U and lost very little performance to the router, yes, AC66U, not AC88U, not an error in typing
Looking for either configuration ideas or to see if others have had the same experience, or not….
Thanks