Yes I was a bit flippant there 
If the average input throughput is faster than the output you are screwed. However if the average input is the same or less than what the output can handle you can buffer the data.
Let's say you have an NMEA string of 100 chars occurring every 100mS. That's an average of 1000 chars a second so your output has to match that or better.
There are usually 10 bits per byte when transmitting so 1000 bytes per second == 10000 bits per second so (if we ignore any overheads) your output stream has to have a baud rate of at least 10000bps.
So the actual input baud rate is fairly immaterial, you need to know the number of bytes received per second.
If the above conditions are met you write some simple ring buffer code to hold the characters while they are waiting for transmission.
Rob