In voice recognition system, how to match digital input to a given word without seconds delay?
There is a massive range of possible sounds that a human might recognize (or mis-recognize) as the same word in their language.  The problem is searching a large dimensional space representing all those possibilities (continuous variations of spectral and timing differences) for the closest match, perhaps also given some context around the search target for the most probable of multiple near matches.  This is a computational complexity problem that can sometimes be reduced in time by more computational power (a 1000 processor server or supercomputer might be able to do it in less than "seconds" of delay).

Or you can search over less degrees-of-freedom with less robustness or accuracy, and perhaps miss most possible word matches.