This thread is representative of several threads that have been started on ASR, where the thread begins with someone asking, "Can _____ be measured?" Or, "Does _____ really matter?" Or something similar. The common theme in these threads is that the question is concerned with some nebulous audio property that is known by some word, but isn't adequately defined. If the property hasn't first been adequately well defined, then there isn't even a good reason for two or more people discussing the property to assume that they are even talking about the same thing. And there is plenty of evidence in these threads that people don't agree on exactly what the property is. The only thing on which there is strong agreement is the spelling of the word they use to refer to the nebulous thing. This is a fundamental problem with debates that take place among audiophiles using web forums. The ensuing debates are pure folly. The occasions where the debates are not pure folly, where there is a strong substantive basis for the debate, are the occasions where the debate has proceeded from a clear, universally-accepted definition of the thing being debated.
The concept of "speed" hasn't any substantive role to play in amplifiers, or in any audio equipment for that matter. The reason is that there are two discernible ways to interpret the concept, and with both, there is a better perspective. One interpretation is with the time rate of change for the signal voltage. This is probably the more obvious interpretation, but it is frivolous, because of the absolute mathematical correspondence between frequency response and the time rate of change of the signal. The question, "Is the amplifier's limitation on the time rate of change as great as it needs to be?" is entirely equivalent to the question, "Does the amplifier's frequency response extend as far into high frequency as needs to? These two questions are in reality the very same question. They are two ways of looking at the exact same thing. The other interpretation is with the phase shift that occurs within an amplifier. Since phase shift can be interpreted as time shift, it is conceivable that some people would think of an amplifier with minimal phase shift as a "fast" amplifier. I doubt if there are many people who think of amplifier "speed" this way, and the only reason I mention it is to underscore the fact that when the terms are not very well defined, two people using the same word might not even be talking about the same thing.
When the audio property of interest is nebulous and inadequately defined, the only questions that are meaningful to ask are questions concerned expressly with the definition. What is the definition? Is the concept adequately well defined? How should it be defined?
