we have done some testing as well and can confirm that corona proxy workflow is a bit slower than using vray proxies or regular mesh.
Scene. 6 trees, 4M each.
Growfx version / With FP
303s / 350s to load
Just Mesh Version / With FP
8s / 44s to load
Just CoronaProxy / With FP2s / 72s to load
Just CoronaProxy Version Full Mesh / With FP
32s / 79s to load
Just vrayProxy / With FP2s / 2s to load
Just vRayProxy Version Full Mesh / With FP
18s / 64s to load
Just Xref Version / With FP
10s / 47s to load
Delay is caused by reading the mesh of the proxy. It's needed to build the point-cloud and to calculate the proxy size.
V-Ray proxy is much faster because it returns an approximation (the same used for the viewport), not the full mesh.
We will contact corona team to inquire about possible solution.