Too weak for a full symmetric 1Gb fibre connection, yes. But ARM-based routers with twice the clock speed of those MIPS cores and significantly better performance per clock and much bigger caches have been around for years, and are pretty good at handling SQM for the DOCSIS-based connections that need it most.
I benchmarked my cheap ($50) Walmart OpenWRT router that has a MediaTek MT7622 (dual-core ARM Cortex-A53 processor clocked at 1.35GHz); SQM is totally usable below 600Mbps or so.
Last i know was that rpi‘s can barely handle one gig but not the effectively two for duplex. Not to mention that the ethernet port (singular) runs over usb
On the last pi, the eth port is on a pci express bus. For routing usage, you will need to either have to add a usb3-eth adapter or run "router-on-stick" with a single eth port and VLAN.
Or, you could also look at the NanoPi R4S or upper model with multiple eth and 1 Gbit capable SQM.
