Does it happen in the build or only in the Editor? I have a similar issue in the Editor. I tried profiling and I found that GFX.WaitForPresent is responsible. According to the comments [here]( Gfx.WaitForPresent page-7) it seems that render thread is waiting for the GPU. I did some more tests and it seems that it is only happening when materials in my scene are using too many too big textures. It is easy to reproduce with just few PBR materials using 4K textures. Funnily enough compressing them with ASTC seems to help a bit
When I build the project with those textures and I run it on an iPhone XR everything seems to be running smoothly.