"bad" latency is very subjective - what's good for one application is horrible for another.
You also need to factor in what you actually bought. If you're expecting 500us latency on a SATA drive, you're going to be disappointed, all day, every day, and there's nothing you can do to achieve that latency target.
Survey your users and ask them to tell you when latency is acceptable and use that as an initial baseline. If they haven't be complaining for the past week at 5ms latency, don't set your target at 0.5ms. If you're getting 1ms latency today and they're screaming bloody murder, then you've got some serious investigation to do and perhaps user education on what is realisticallyo possible, assuming you don't have a blank check to write to your vendors.
