Hello everyone !
I only discover this thread after months of research about this issue...
There is hope ! Since the last week, I'm testing the latest kernel updates from amlogic ([3.14] Merge kernel updates from Amlogic Nougat 2017-09-28 SDK by codesnake · Pull Request #70 · LibreELEC/linux-amlogic · GitHub) and I didn't had any slowdown since that.
Maybe it's just by luck, I'll continue my tests for some days and keep you informed.
There is also another option, hyperion.ng introduced a new capture mode for the amlogic grabber. It's done directly via GE2D / amvideo and not by amvideocap anymore.
It needs additional kernel patches : Commits · Arakmar/linux-amlogic-le · GitHub
It works just fine for the hardkernel kernel. But I haven't manage to get it working on the Librelec amlogic kernel, it captures just a black screen.
I need to investigate that a bit more when I'll have time.