High vm's write latency

Hello,

We have experienced problem with high vm's write latency on vmfs datastore (4x4TB RAID 5).The GAVG parameter on vmhba and disk has 2-10ms, but on vm view our vms has from 20 up to 60ms write latency. On another host  (6x4TB RAID50) with higher load we have write latency from 0 to 5ms for all vm's running on it. What can be the problem if GAVG is at acceptable level but vm write latency is high?