While it is true that the actual data throughput of 802.11b is less than 11mbps it can be faster than 6mbps.
There are certainly factors that have a negative impact on throughput but under controlled circumstances I've measured actual data throughput up to 7.2mbps on 802.11b. If you actually have a solid 11mbps connection (no interference, not too far from AP) this is really achievable. Of course if there are signal quality issues this number will drop.
802.11b could be but does not have to be the bottleneck in this case.
If it's just a few computers at relatively close distances to the AP it could be just fine.