The double-escapement action in modern grands is capable of repetition at virtually any rate up to the limits imposed by the inertia of the action parts. So why do so many grands repeat so unpredictably? The answer is technical, but if you know a little about grand piano regulation, you should be able to follow this article. If you have Pianos Inside Out, see "How the Piano Mechanism Works," pp. 76–82, and "Regulating Grand Action," pp. 166–179.
For efficient repetition, the action must be regulated precisely, with both escapements (letoff and drop) occurring very close to the strings. The repetition spring has to be adjusted to provide enough upward push to the hammer to allow a quick repositioning of the jack under the shank knuckle during repetition. Unfortunately, in many pianos the letoff and drop are regulated inexplicably far from the strings, and it takes extra finger force to propel the hammer past those escapement points.
The repetition is further enhanced by the backcheck, an ingenious device that prevents the hammer from traveling too far down after a strike and possibly bouncing back to the strings. By catching the hammer fairly close to the strings, the backcheck not only reduces the time the hammer spends moving away from the strings, but it reduces the time the hammers needs to get ready for a restrike. The closer the backchecking is regulated to the strings, the faster the repetition. However, the backchecking distance must not be too short or the repeated notes will play only very softly and you won't be able to vary their dynamics.
For optimal repetition all parts in the action and keyboard must move relatively freely, without excessive friction. Key bushings must be free and key pins smooth and well lubricated. Balance holes must be free enough for the keys to rock without resistance, but not so free that the keys would have perceptible fore and aft movement. Balance mortises must be free of debris and mortise floors or shoes relatively thin. Even the friction of a seemingly unrelated part of the mechanism—dampers and damper underlevers—must not be excessive or the repetition will suffer.