Strictly speaking, there isn’t, though I can see (and hear) why you say there is. I’ll try to explain what is happening, and then we can look at what can be done.
Reverb effects are basically modelled on the way that sound bounces around within an environment. It is similar to an echo effect in that it models sound bouncing off walls, but rather than just a few distinct echoes a reverb effect is mad up of hundreds or even thousands of “reflections” of the sound, mixed in with the original sound.
Looking in more detail, imagine if you are standing at one end of an empty room and someone else is stood near the other end. They clap their hands. The first thing that you hear is the direct sound from their hands, travelling across the room to your ears. Sound (obviously) travels in all directions from their hands.
Some sound will come towards you, but rather than a direct path, will travel slightly downward, bounce off the floor, then up to your ears. Because of the finite speed of sound, this reflection will clearly arrive at your ears a little later than the direct sound, but as it is almost a direct path and sound travels very fast, it will only be delayed a tiny amount. Some sound will go the opposite direction, bounce off the far wall, and then to your ears. This sound will obviously arrive at your ears just a little later. Some sound may bounce off the left wall, then the right wall, then the ceiling, then the left wall, then the wall behind you, before finally arriving at your ears and this sound will clearly be delayed much longer before you hear it. The overall effect is that after an abrupt sound (such as a hand clap), the reverberation effect will build up as the echoes arrive, then gradually die away as the sound disperses and is absorbed by materials in the room. The initial “delay” that you hear in this effect is not actually that the effect has been delayed, but is the time it takes for the reflections to “build up”.
For real (natural) reverberation, the Initial direct sound will be delayed by the time it takes for the sound to travel from the sound source to your ears (about 343 m/s). For artificial reverb effects we normally disregard this delay as, typically it will be very short (about 3 ms if the sound is one meter away), and we don’t want to mess up the timing of the sounds in the recording.
The time for the first reflection to arrive will depend on the size of the room, where the sound is coming from, where the listener is, and the path taken by the sound before it arrives at your ears. Lets say, for example that you are in the middle of a room that is about 8 m across and has carpet and a sound absorbent ceiling and you are listening to someone that is 1 meter in front of you. The direct sound will take about 3 ms to arrive, but the first strong reflection will be from one of the walls, 4 m away. The sound has to travel from the sound source, to the wall and then bounce back to your ears before you hear that first reflection. That’s a round trip of around 8 m, which will take around 23 ms to arrive - about 20 ms after the direct sound. What we can say from this is that our reverb effect is modelling an environment in which the closes strongly reflective surface is about 4 m away - a moderately sized room for a musical performance.
In this reverb effect, all the settings are based on adjusting properties of “one room”. The effect as it currently is, is based on one “room model” that was devised by Roger Dannenberg (the creator of the Nyquist programming language). What I think we need rather than an adjustable pre-delay, is more than one room model. There is no reason why this can’t be done - in fact we could model reverbs on any size room that we like. I think this is an excellent idea (based on your observation). Thanks for the feedback - I’ll implement that in one way or another in the next version.