What happens when a battery is low? Will the shifts slow down? Will the shifter work intermittently?

Shifting performance is unchanged regardless of remaining battery charge. The components do provide indicator lights for battery life. After each shift, a light on each component turns on:

 

Rear derailleurs:

Green LED: 15-60 hours of ride time remaining

Red LED: 5-15 hours of ride time remaining

Flashing red LED: Less than 5 hours of ride time remaining

 

Front derailleur:

Green LED: 22-90 hours of ride time remaining

Red LED: 9-22 hours of ride time remaining

Flashing red LED: Less than 9 hours of ride time remaining

 

Shifter/Brake lever:

Green LED: 6-24 months

Red LED: 1-6 months

Flashing red LED: Up to or less than one month

 

See the SRAM RED eTap System Overview video for a visual of the indicator LEDs.