Specified Cubemap setting in Reflection Captures taking really long to load

We are having an issue, where when we specify our own cubemap textures rendered and converted to textures through the scene capture actors inside our spherical reflections, our game takes about 5 minutes to load. When we switch the spherical reflections to use “capture scene”, it takes 5 seconds. We have about 10 reflection spheres and the maps are HDR compressed (1024X1024). Any pointers would be great.