Why is it important for a hash function to be fast?

It matters that a hash function is fast, because it is called on every operation with a hash table: inserting, searching, updating, and removing an element.

#If the function is slow, everything suffers

1. Every access to the table requires computing the hash, so even "O(1)" access turns into a long process.
2. The hash table stops being efficient: its advantage is speed, and that is exactly what gets lost.
3. With a large volume of data (millions of keys), even extra microseconds spent computing the hash add up to huge delays.

#Goal

A hash function must be simple enough to run almost instantly, and good enough to distribute keys evenly (minimal collisions).

#Conclusion

A fast hash function is the foundation of a hash table's performance. If it is slow, the hash table loses its point, it would be simpler to just use a regular list or tree.