Y'all might be interested in using NeoSWSerial instead of SoftwareSerial, if the baud rate is 9600, 19200 or 38400. It doesn't block interrupts for the entire time a character is being received. And unlike SoftwareSerial, NeoSWSerial can transmit and receive at the same time.
There is software to use the USI as a serial port here. That would be much better than any software serial library, but it can only use pins 0 and 1.
Cheers,
/dev