Complex sentence: Second order Linwkwitz-Riley crossovers (LR2) have a 12dB/octave (40 dB/decade) slope, and they have constant group delay, resulting in good transient response.
Split into simpler sentences: Second order Linwkwitz-Riley crossovers (LR2) have a 12dB/octave (40 dB/decade) slope. They have an almost linear phase response, that means they have constant group delay, resulting in good transient response.