FMOD has to schedule each sample to be played in advance of when it actually plays. When you change the value of a parameter, FMOD has to change which samples are scheduled to play, and it needs a small amount of lead time to do so. This is the source of the latency you are hearing. We have endeavored to minimize it, but certain amount of latency when changing parameter values is expected and unavoidable.
By default, FMOD schedules 8192 samples ahead; at 48kHz, this translates to around 170.7ms of expected latency.
Streaming assets incur around 85.3 ms of additional latency due to the need to buffer the stream. If the affected assets are streaming, setting them to not stream should reduce latency by around that amount.