This thread shows that most of us consider "expressiveness" (not speed!) to be the opposite of "sluggishness."
The elements used in expression have been mentioned: legato/staccato/détaché, crescendo or diminuendo on one note, breaking the tune into phrases, with each phrase having a rise and fall within it, etc.
What stands in the way of doing this?
There's an old English adage that says, "A little learning is a dangerous thing."
Applied to music, this means that a learned, experienced conductor can look at a score, and say, "We need a crescendo here, and a rallentando there; this phrase is a transposition of the phrase before, so we should play it softer; and this last chord should just die away to nothing."
On the other hand, the musician whose learning is limited to knowing what the notes on the stave are called, and where to find them on his instrument, can't see what a conductor sees, and tends to just play one note after the other. That is what you describe as "sluggish."
There are two ways out of this trap: either you study music theory and practice at a high level until you can read a score in your head - or you treat the reading of the sheet music onto your instrument merely as a way of getting a tune into your head, and then play it out of your head.
As MileFule wrote, " If you were humming or "pom pom pomming" the tune to yourself in the shower, the expression would come naturally." When you've reached this point with a tune, you can start playing it on your concertina.
Another impediment to expression is a stiff posture. The beginner can be so taken up with pressing the right buttons and moving the bellows in and out, that he thinks that the fingers and arms are all that is involved. But watch any professional instrumentalist - any instrument, any genre - and you'll see that the expression he or she is putting into the playing is mirrored in their body language. Or rather, their playing mirrors the body language that expresses their intentions. Simple example: most musicians lean forward to build up tension, and lean back to ease tension - and this automatically results in playing louder and softer.
So sit on the edge of your seat with both feet firmly on the floor. It really does help!