How do you explain slow/lazy sound?

How does one go about explaining slow/lazy sound? I mean in technical terms in relation to the soundwaves. I know the music itself is not slower but somehow the sound just sounds slow. To the point of mild frustration...

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.
Slam or attack is in the upper midrange 1 to 3 Khz (when a drum stick hits the skin or a piano hammer hits the strings). If you cut this upper midrange then it will sound laid back (many speakers do so and sometimes deliberately so). Another effect is bass. Good transient reponse from critically or over-damped speakers will mean that transients do not last any longer than they should (sealed subs and speakers with no reflex ports are often the "tightest" and cleanest sounding). Ported designs with amazing bass for a small box are often culprits that give a lazy or "one note" bass sound. Box resonances and woofer resonances (ringing) will also muddy or slow the sound by adding distortion (boosts especially in the 500 to 800 Hz range will add warmth).

Note that many speakers (especially two ways) use a 6 or 7 inch woofer to cover bass and midrange - inevitably many of these designs beam above 1 Khz (the woofer is just too big) and to varying degress give a laid back sound. Beaming means they excite just the sweetspot with a flat response whilst the rest of the room reflected off axis response is heavily down until the tweeter kicks in between 3 and 5 Khz adding that unatural "hi-fi" sound where the lead acoustic guitar may be just a wee bit too clear to be convincingly natural - kind of like there is a microscope on it...again some speakers are deliberately designed this way (remember as much as half of what you hear is reflected)

Another factor is dynamic response - this is very counter intuitive. If you compress the sound and it lacks dynamics then the sound actually becomes perceptively louder or edgy or snappy. This is because you have raised the root mean square energy (energy over time). Teh sound can be very tiring and fatiguing and is common on pop CD's today but especially in Punk and Heavty metal such as Metallica or Green Day. A highly dynamic system with a highly dynamic recording (contrary to what you would expect) will sound softer, more natural and relaxed, effortless and less fatiguing even at higher SPL.
I asked that question because I set up a second turntable just for fun... My main rig is a Musichall 5.1 with Goldring 1042. I bought a Pioneer PL-12D on Ebay and installed a Shure MX97SE just for fun... And the Pioneer didn't sound bad, but the only way I could describe the sound is "slow". I was just curious why it sounded that way and what is the physics behind it.

Possibilities.Inadaquate conditioning/isolation for the preamp. Likewise for the digital.
Incorrect absolute phase is a possibility.
Assuming you are only hearing this "slowness" with TT and that speed of TT has been adjusted to strobe, then the "slowness" may be slurring caused by transient speed instability endemic with cheap belt-drive TTs.
This is primarely about the equipments powercircuits ability to "breathe". Powercircuit starts with powercords and ends in speakers voicecoils. Can`t be overrated!

The problem has two sides; speakers and amps. First we have amps with tiny underdimensioned powercircuits struggelig to beathe through heavy passages. Then we have speakers with even more underdimensioned passive filters threathening to shortcut the poor amp.
And on top of this the poor amp is given some tiny overpriced stranded or flat "wires" as a extension of it`s allready tiny powercircuit.

Tips: Avoid complexe passive filters, avoid speakercables, go for monoblocks. Powercords w.o extensions, heavy solid-core speakercables.