HashMap vs array performance difference in following approach

To solve Dynamic programming problem I used two approaches to store table entries, one using multi dimension array ex:tb[m][n][p][q] and other using hashmap and using indexes of 1st approach to make string to be used as key as in "m,n,p,q". But on one input first approach completes in 2 minutes while other takes more than 3 minutes. If access time of both hashmap and array is asymptotically equal than why so big difference in performance ?