3 things to consider in a router, the OS, RAM and CPU. Just like a PC. Mikrotik's RouterOS, is very stable. You can throw torrents, vlans, qos, filters, etc.. at it and it just keeps on going.
Next is RAM, everything uses a bit of RAM. BitTorrent, as you know, opens a lot of connections so having enough RAM is crucial, pun intended. The RB750Gr3, for example, has 256MB which doesn't seem a lot but is plenty, even for hard core home use.

Looking at my router's connection list, it says it has space for another 472000 connections.
Next is the CPU which determines how much bandwidth it can route. 100mbps is easy peasy for the RB750Gr3 with or without hardware accelerated NAT. It'll also determine what else you can do with the router like vlans, firewalls, QoS, etc...
Having said that, with a simple config, the RT-AC66U should be able to handle 100mbps with no sweat. Just make sure you have hardware NAT acceleration turned on and that you're not using any features that automatically disables HW NAT. I believe QoS may do that but I'm not very familiar with Asus. What firmware are you using with yours?