Let's talk about time reversing a layer first. If you have a clip that's 30 seconds long and you have trimmed that clip to 4 seconds and you time reverse the layer it's going to take about 4 times as long to render or ram preview as it would when rendered normally. This is because Time Remapping is calculating the length of the clip and the current frame every time it renders a new frame. This takes a long time. If you applied Time Remapping the same clip and then used the keyframe assistant to time reverse the keyframes then these time waisting recalculations go away and every frame is defined by the keyframe data. Renders will be about the same speed as rendering the clip without any effects or transformations. I can't remember when I last used Time Reverse Layer. It's been working like this for as long as I can remember. Maybe it's time for a feature request or a bug report to get someone on the AE team to re-do the math behind the effect.
Another thing that could be slowing down your render is the comp size. You are rendering a huge movie at a completely non standard size. I'd render something like that to an image sequence. Second, you said that you have a grid of 100 pre comps. How big are these? If you've scaled 100 pre-comps that are full HD down to fill your comp then you are asking AE to process 100 HD layers per frame and make those interpretations. The render is going to take some time. If you throw scaling way down, motion, and any other effects on top time reversing a layer you are in for some seriously long render times.
good to know there is a faster way to reverse.
I didn't know reversing was a seperate process to time remap keyframes.
Does that mean if you have less keyframes, you have shorter render times, in general?
The rest is by the by, as I was reversing a clip not a comp, if set proxy it's job.
Just explained the comp to show that it did require a fair bit of rendering, which i did not think time reversing would.
Time reversing is recursive. You can write recursive expressions that will even slow down simple things like position transformations. The problem starts with AE's inability to look at more than one frame at a time. Temporal effects require recalculation of different slices of time and then combining the result with the current frame. Time Remapping doesn't do that unless you enable frame blending. Time remapping simply gives a frame value to each frame and that's why it is so much faster than time reversing a layer.