I do not find Darcy condescending; in fact I find him very gentlemanly here. Here is why:

First, we know Darcy is thinking of talking to Lizzy but feels a need to listen in on her conversation to figure out how to talk to her. She catches him and (on a dare from Charlotte) confronts him, one might say mildly attacks him verbally.

Darcy's response is to talk by rule: he deflects Lizzy's "attack" by ignoring the merits but tacitly admitting the substance, that he had overheard. He compliments her energy; so far he is figuratively "talking about the weather," having a conversation that reveals nothing personal about the participants. But then he offers a slight challenge of his own, tentatively moderating his compliment. IMO Darcy is still trying to learn how to talk to Lizzy. Lizzy's "severe" comment shows that she notes the challenge. I wonder here whether Lizzy's reaction is "overstated" to tease Darcy back or whether she is doing this lest she "grow afraid of him."

"He has a very satirical eye, and if I do not begin by being impertinent myself, I shall soon grow afraid of him" (Ch. 6)

At this moment Charlotte steps in to turn the conversation. (She got Lizzy into this encounter, so perhaps she feels obliged to get her out.) I agree with Robbin that this whole scene shows how delightfully close Charlotte and Lizzy are, and it shows something of their relationship early in the story. The difference in age seems to make Charlotte something of a mentor in terms of experience (whether Lizzy sees this or not) but the two remain equals intellectually. I think in terms of wit, the two young ladies are a match.

I also think that Charlotte is doing what she can to maximize Lizzy's chances with Darcy, whether or not she expects to have any success. Charlotte knows Darcy will never be interested in her and can see Bingley is already interested in Jane, so why not see what she can do for her friend. Charlotte never fully bought into Lizzy's first impressions of Darcy, and by suggesting that Lizzy now play the "instrument," she seems to be trying to improve Darcy's opinion of Lizzy. No doubt Lizzy senses this too, which is but one reason she initially demurs:

"I would really rather not sit down before those who must be in the habit of hearing the very best performers."

Lizzy's comment to Mr. Darcy ("keep your breath..."), IMO, relates to that: she knows he has heard better, feels obligated to oblige her hostess, and (in her charming archness) is warning Darcy off teasing her about it. I do not think she feels insulted by Darcy's mention of ladies being energetic about dancing or that this comment relates back to that.

I also note Darcy's later disappointment when Lydia and Kitty persuade Mary to play for dancing.

Mr. Darcy stood near them in silent indignation at such a mode of passing the evening, to the exclusion of all conversation, and was too much engrossed by his own thoughts to perceive that Sir William Lucas was his neighbour, till Sir William thus began...