When the writes of per second up to 3500, the read request time increase by about 80%, why?

About my Aerospike Database : namespace : Good;
sets : language, item, item-transaction,item-skus;
memory: 30G;
nodes : 8 ;
CPU: 4 core ;
when the writes of per second up to 3500, the read request time increase by about 80%,but CPU、Memory and Traffic-network are normal。

Did you check if you are choking on other hardware resources like Disk IOPs, n/w bandwidth? If you are using the default write policy which has

policy.recordExistsAction = RecordExistsAction.UPDATE;

then there would be Read OPs from the disk for every write operations. We ran into similar issue where we got bottlenecked on the IOPs before CPU.