If you only want to perform an analog read every 100ms and consume stuff read from SoftwareSerial at 9600 bps then I don't see a problem. SoftwareSerial is interrupt based so as long as you check for received data often enough to prevent the 64-byte receive buffer from overflowing it should be fine.
All you need is a loop() which: reads a character from the SoftwareSerial port if it is available, and buffers it locally; processes the buffered input when a complete NMEA sentence has been received; performs the analog read if is it time to do that.