Upgrading from the RT-N66U, RT-AC56U, RT-AC68U, RT-AC3100, RT-AC86U to the current RT-AX88U, and RT-AX58U (wired AiMesh node), I can tell you each upgrade was made for
performance reasons.
https://www.snbforums.com/members/l-ld.24423/#info
With the RT-AC88U equivalent to the RT-AC3100 in WiFi performance, the RT-AC88U is at least 2 upgrades ago.
See the link above for more details.
With AC clients connecting at about 40 to 400Mbps previously and now connecting at 866.7Mbps in the most used areas, and the download speeds increased proportionately, the previous AC king (RT-AC86U) has been
beheaded.
With AX clients connecting at 2.4Gbps almost everywhere (wired AiMesh) and hitting above 850Mbps (minimum 650Mbps) and uploads hitting 930Mbps, the improvements are real.
https://www.snbforums.com/threads/intel-ax200-wireless-card.64184/#post-585324
The results above are not just a swap of one router to another. Rather, it is a full and systematic set up to enable the new hardware as fully as possible in the WiFi environment it is being installed in.
This has included upgrading the laptops that I can with AX200 cards. Adding a second router at the other end of the home in wired AiMesh mode. Using (and being able to use) 160MHz bandwidth for the AX routers and all (AC and AX) clients.
My ISP speeds are Fibre 1Gbps up/down symmetrical. The RT-AX88U main router's hardware (RAM and CPU cores) makes better use of that ISP speed more effortlessly than the RT-AC86U ever could. Regardless of wireless performance.
The RT-AX88U with RMerlin firmware and amtm scripts is the best combination of performance, features, security, and ease of use (not to mention the lowest overall cost too) that I have found.
I bought it to test it but kept it because it delivers on (most of) the performance it promises.
If 2.4GHz performance is
more important to you, the RT-AC3100/RT-AC88U
may have an edge. But not by much and overall will be a downgrade over the performance the RT-AX88U has on tap.
HTH.