How do you explain slow/lazy sound?

Slow rise times and slow decay (slew rate) will make square waves into low mounds. Visualize that, with rise time for example, a component that can replicate the leading edge of a 'square wave' can replicate the leading edge of a cannon shot so replicating a square wave is good. However the converse is also true, a square wave which is perfectly vertical (or close) on the back edge, would rob the music of a more natural sounding wave. In fact IMHO the treating of the 'fall' time of the back edge is more important than the slope of the leading edge. It is sort of a finding of the proper degree of slew rate - too little and you have a muddy echo chamber type residual v too fast and the sound is too clinical and unmusical.

Hope that helps a bit.

Try different speaker cables, some that are known for speed (Like Nordost Blue Heaven).
I used HT Pro-9 with a low power SET tube amp and it was
remarkable how slow and syrupy it sounded (I'm using HT with SS and its sounds great!).
Blue Heavens w/SET was night and day difference.

Go to Virtual Dynamics cables...They are quite well know for there speed and Dynamics....

Rwwear...Rise and fall times of all amps are much much shorter than loudspeaker response times.