Debussy performed on several occasions in public, and gave the first performance of many of his piano works. Léon Vallas about the concert 12th of march 1912, where Debussy played som préludes:
[He played] in his usual delicate manner. His playing, so full of intimate feeling and subtle effects, was not heard to advantage in the big Salle Gaveau. [Vallas p. 242]
I have been told he did not always play to advantage in a concert hall. He said that afternoon that many of the Préludes, especially 'Les danseuses' and 'Des pas sur la neige', should only be played entre quatre yeux. [Nichols p. 202]
It could be that Debussy's pianistic ideals didn't suit the big concert halls, which favor the extrovert and virtuosic. Another reason could be that Debussy was very nervous when he performed in public.
Jacques Durand on a concert in a salon in 1889, playing the four hand work Petite Suite:
Debussy was very nervous before sitting down at the piano with me, and had urged me not to go too fast. I promised. But hardly had we begun when Debussy began to hurry; and despite all my efforts, I was unable to hold him back. He was in haste to put this public trial behind him. So I followed the somewhat hectic tempi as best I could, and the work finished with a brio that was, probably, an important factor in the polite sympathy with which the work was finally greeted. [Nichols p. 30]
The concert critiques are however very interesting, considering they were contemporary accounts, normally written by someone not acquainted with Debussy personally.
The critic in Le Monde Artiste on the concert March 1913:
M.Debussy, in person, played three of his Preludes. The first, 'Bruyères', has no significance at all; the second, 'Feuilles Mortes', has very little more; the third, 'Puerta del vino' ... is an improvement on the two first pieces, without being at all remarkable. But what was remarkable, was the behaviour of a section of the audience. There were enthusiastic cheers and numerous calls, and some girls even threw flowers to the celebrated composer, who picked them up with evident pleasure... M. Debussy is undoubtedly a man of great talent and a very clever pianist. But he must realize himself that his trifling little pieces do not deserve the same ovations as the masterpieces of Gluck, Beethoven, or Mozart. [Vallas p. 244]
On the 12th of march 1912 Debussy performed some of his preludes in Salle Gaveau. The critic accused him for holding a continuous pianissimo throughout the performance [Vallas p. 243]
Another critic in La Critique Musicale praised the pianists "ease and fluidity, and his insinuating, though rather monotonous tone. [Vallas p. 245]
After a concert 21st of march in Salle Gaveau, where Debussy accompanied Ninon Vallin in the Trois Poémes de Mallarmé, and played Children's Corner, Freuilles mortes, La Cathédrale engloutie, and La Fille aux cheveux de lin [Vallas p. 244], the same critic wrote:
I confess that I prefer his playing to that of any other pianist. His restraint and discretion make one forget all considerations of technical dexterity. Thus the naked soul of the work is placed before the audience; its action is direct; the interpreter is lost sight of. [Vallas p. 245]
Léon Vallas wrote a critique in Revue Française de Musique on the same concert:
The performance was not lacking either in interest or monotony. M. Debussy gave us a charming rendering of his 'Preludes' which sound like clever pastiches written in the manner of... Debussy; he also played in a dim, veiled tone, that was at times almost inaudible, some accompaniments of songs which Mme Vallin-Pardo, skilfully muting her powerful voice, rendered in a soft unchanging murmur. This subdued colouring, evidently desired and insisted upon by the musician, is in keeping with the general tradition that composers are apt to be the least successful and least convincing interpreters of their own works. The extremely elegant audience enjoyed a pleasant doze during the music, and woke up to join in the unanimous applause... [Vallas p. 243]
Louis Laloy, as a critic in Comædia after a concert with A. Hartmann, 5th of february 1914. On this concert they performed a violin sonata by Grieg, together with some pieces by Debussy:
The power of magic will be realized by all who have ever heard his marvellous playing; the sounds seem to be produced without any impact of hammers or vibration of strings; they rise up into a transparent atmosphere where they unite without merging, and then dissolve in iridescent mists. M. Debussy puts the keyboard under a spell, the secret of which is unknown to any of our virtuosi. [Vallas p. 245]
The young Darius Milhaud:
I had also the possibility of hearing Debussy in concerts – he played sometimes when he accompanied certain songs – and I was always extremely moved by the tenderness with which he touched the notes; he played magnificently. [Nichols p. 243]
Debussy seldom played in public. But when he did so, at the Société Nationale or the Concerts Durand, it was an excellent demonstration of his principles. Once at the Salle Erard he played several of his 'Préludes'. As usual, an attendant raised the lid of the concert grand. But when Debussy came on, the first thing he did was to lower the lid. 'C'était pour mieux noyer le son,' he said. Drown the tone... how wonderfully he did just that in 'La cathédrale engloutie.' It was truly unforgettable. [Nichols p. 162]
Marguerite Long confirms this curious detail [Long p. 39].
Dr. Richard van Rees from the board of Concertgebouw, on the concert in Amsterdam 1st of march 1914:
Debussy on the rostrum appeared ill at ease. [...] success attended his performances on the piano, even though technically these were far from perfect. [Nichols p. 232]