To the best of my knowledge, the OpenWRT table of supported hardware (or the older version) is the place to start. There's also a DD-WRT version if you want to compare. These tables list supported devices and detail their hardware capabilities, including CPU platform, CPU frequency, amount of RAM and flash memory capacity.
Most devices in these tables are older and will have a CPU speed of ~200MHz. Glancing through the OpenWRT table, a few more recent devices jump out at me as having potential. This is a cursory look, though, and it's hard to tell how one CPU stacks up against another without more detailed research.
- D-Link DIR-825, 680MHz CPU (Atheros AR7161), 8MB flash, 64MB RAM
- Linksys WRT350N v2, 500MHz CPU (Marvell 88F5181L), 8MB flash, 32MB RAM
- Netgear WNR854T, 500MHz CPU (Marvell 88F5180NB1), 8MB flash, 32MB RAM
- Ubiquiti RouterStation and RouterStation Pro, 680 MHz CPU (Atheros AR7161), 16MB flash, 64MB RAM (128MB on the Pro) (might have to add your own wireless miniPCIe NIC(s))
Additionally, the Work-in-Progress section of the table lists some other interesting devices:
- Linksys WRT400N, 680MHz CPU (Atheros AR7161), 8MB flash, 32MB RAM (watch out for useless template info at the link)
- Linksys WRT610N, 300MHz CPU (Broadcom BCM4705), 8MB flash, 64MB RAM (WIP)
- Mikrotik RouterBoard RB-450G, 680MHz CPU (Atheros AR7161), 512MB flash, 256 MB RAM (!! ... awww, no wireless... )
- Netgear WNDR3700, 680MHz CPU (Atheros AR7161), 8MB flash, 64MB RAM
Those seem to be the beefiest devices listed, but I'm not familiar with any of them off the bat. It's a start for your research, though. From here, you might look into the capabilities of the Atheros CPU that the fastest of these devices use, and see how it compares with the Marvell CPUs used by the others.