Expect is usually much faster than any human. However, certain behavior can force Expect to be slower than it could be or even worse, to fail altogether.
Some programs produce an astounding amount of output. Graphics programs are one example, but even programs that simply list files can produce a flood of output. The rate is not a problem. Expect can consume it and make way for more very quickly. But Expect has a finite amount of memory for remembering program output. By default, the limit is enough to guarantee that patterns can match up to the last 2000 bytes of output.
This is just the number of characters that can fit on a 25 row 80 column screen. When a human is viewing a program producing a lot of output, everything but the last 2000 or so characters scrolls off the screen. If a decision has to be made, the human must do it based only on those last 2000 characters. Following the philosophy that Expect does what a human does, Expect effectively defaults to doing the same thing: throwing away everything but the last 2000 characters.
This may sound like a lot of information can be missed, but there are some ameliorating factors. In particular, if an interactive program produces a lot of output (more than a screenful) and wants to make sure that everything is seen, it will present the user with a prompt (e.g., "
more?“). Expect can recognize this too.
The behavior of Expect to forget (i.e., throw things away) does not mean that Expect ...