Can you elaborate what latencu issue you have? Delay, distortion?
I guess you mean you use 128 samples buffer size.
Please also provide the used samplerate, and if you have USB buffer settings those too.
Please also provide your computer specs.
Is it in both standalone version and as plug-in you have the issue?
If the issue is in your DAW, then what DAW are you using and what is the soundcard settings for your DAW?
EDIT: It's true that Piano V can use a lot of CPU power. For instance if you play lots of notes - especially in the low range - while using sustain pedal.
A buffer at 128 samples can need a very good computer and soundcard - especially if you play in demanding ways. The higher samplerates you use in addition to this, the better your gear has to be.
You can set Piano V to use multiple cores if you have'nt and has multicore CPU. That can help some.
Arturia i suggest you put in a parameter to select the max number of voices to be used. This also can help on how much CPU power is used and needed.
I can't seem to find such a parameter.