K-shortest path query becomes slow with k>3


I encounter a strange behaviour of k-shortest path query on my dgraph cluster. This query is pretty fast for k<=3. However, when I set k>3, the sever latency increases from <1s to 10~15s.
Here is my test result:
k server lantency
<3 <1s
4 10~15s
5 10~15s
10 10~15s
30 10~15s
50 10~15s

Could you please give some suggestions for this problem?