Today I learned that new species can become established in a matter of centuries.
There are several related species of mouse on some island who evolved in 600 years, about.
There is a species of mosquito in the London Underground that have evolved in 150 years.
The thing that can really speed up evolution is a genome-doubling event, or something similar if the population has already undergone a genome-doubling event.
Partial aneuploidies, in which only some chromosomes occur in increased or decreased numbers, tend to be highly disadvantageous. Whole-genome-doubling events are much less likely; but also less likely to be detrimental —— or likely to be less detrimental — if they do occur.
Then if one of a duplicated chromosome mutates, there’s probably still an unmutated version still doing its old job the old way. That makes mutations less likely to be fatal.
The real creator of speciation is (I think) splitting the population into two or more groups who will then be isolated from each other. Usually the splitting and isolation are geographic; but it could be something else.