Just to make things clear, the observed transition latency is not something we are addressing as a bug, so there is no easy fix to wait for. Allow me a moment to explain the latency you’re seeing.
When something is put on the timeline, we evaluate ahead of time to ensure playback is sample accurate and smooth. If streams are used anywhere, this will add latency because we need to ensure they are buffered up ready for playback. So my first tip is don’t use streams anywhere in an event to reduce its latency. This latency is called scheduling delay.
Secondly, FMOD internally updates every 20ms unless you change this setting via FMOD_STUDIO_ADVANCEDSETTINGS. To ensure no gaps in timeline playback we schedule double that time in advance, so 40ms. When you have a condition on the timeline it is evaluated ahead of time, this is probably the latency you are seeing. To reduce this latency you can change the Studio update rate to 10ms, halving the latency. This latency is called scheduling lookahead.
If there is a particular EventInstance where you want to override these settings take a look at EventInstance::setProperty with FMOD_STUDIO_EVENT_PROPERTY_SCHEDULE_DELAY and FMOD_STUDIO_EVENT_PROPERTY_SCHEDULE_LOOKAHEAD. Use these with care though, as I mentioned above setting too small could cause sample inaccuracy.
If the above doesn’t help you may need to consider engineering the Event differently.