In general the hash function generates a sequence of table slots and the these values are tested in order until an open slot is found. The time for an insertion depends on how many probes are required to find an open slot. A success makes the same sequence of probes as was made when the object was inserted. Successful search are surely less costly than unsuccessful ones, because the search stops as soon as the item is found. Intuitively, cluster tend to grow, since if a cluster has size p, a hash is p times more likely to hit it than to hit any given open spot.
Big clusters present big targets and grow with each hit.
Worst possible cluster fills half the table. Prevents simple linear clustering, but can still produce secondary clustering, which is not so expensive as primary clustering. Otherwise, find would not work.