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The reason for ISR "low" performance, they (and their predecessors) were really designed for being feature rich while working with "low speed" serial WAN links. Traditionally, 75 Mbps would be a rather fast WAN link.
Today WANs, running at LAN like speeds, often are LAN like media hand-offs, i.e. Ethernet. Cisco does have a line of MetroEthernet L3 switches, feature poor compared to ISRs, but they offer LAN forwarding performance.
In any case, the "fastest" 2900, the 2951, is Cisco recommended for up to 75 Mbps WAN bandwidth. As you note, this allows for "concurrent services". ISR performance depends very much on average packet sizes and how the router has been configured. At the other extreme, the 2951 is documented as being able to push 5 Gbps, with no services and with maximum size packets. Cisco recommendations are conservative, which means a 2900 series might be perfectly fine on your 100 Mbps link, or might not too. Much would depend on you configuration and what your traffic is like.
Cisco recommends the 3925 for up to 100 Mbps of WAN bandwidth.
I've attached a Cisco whitepaper, which provides much more information about ISR performance.
