This command does just what it says - disables fair queueing on the interface, which results in interface running FIFO. With older not-so-powerful routers like 2500 it was often necessary to disable fair queue on 2Mbps serial else router could be overloaded. Today this is hardly issue (many even run more sophisticated things like Class-Based Wheighted Fair Queue and Low Latency Queue), but if you see that your router has high CPU load (80 and above) _and_ 'sh proc cpu' shows that significant part of it spent handling interrupts (second number after / sign), then disabling fair-queue may help (but not necessarily).
In reverse, it's recommended to enable fair-queue on links slower than 2Mbps to provide more fair split of link bandwidth to the users.
