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® 2x11 System Overview video for a visual of the indicator LEDs.