Computers work faster than people who click manually.
Sounds like the guy who wanted an event to be fired for each audio sample and required 44100 samples per second. Interestingly, qt signals maybe even fast enough for just that.
Signals are good for rare, unpredictable events. If you have a firehose worth of data coming in, you're not sitting around waiting very often. Your data probably also has some commonality to it, and can be handled in bulk. If you have 100K events coming in, they're probably events of the same type.
Just like if you're writing gigabytes to a file, a program written with the most minimum amount of intelligence isn't going to do it one character at a time.
Also to process important incoming messages on the same thread as the main event loop - where other things like user input, application drawing is handled, is kind of a bad idea.