I believe the object & counter you're looking for is system:sys_avg_latency. I don't have an ONTAP 7.3.7 system handy, but you can determine if this counter is available by issuing the following command on your controller:
ONTAP> priv set diag
ONTAP*> stats explain counters system sys_avg_latency
Counters for object name: system
Name: sys_avg_latency
Description: Average latency for all operations in the system in milliseconds
Properties: average
Unit: millisec
Base Name: total_ops
Base Description: Total operations per second
Base Properties: rate
Base Unit: per_sec
If this counter is available in ONTAP 7.3.7, you can collect it in a couple of ways:
1) using the stats command at the CLI of the filer,
2) Using a tool like Performnce Advisor or CMPG to collect, save historically and graph the data, or
3) Using the NetApp Manageability SDK you should be able to make an API call to collect that performance object & counter. I don't have the details on which API function is used to make this call.