Android NDK - Multithreading Slowing Down Rendering?

I have an Android app with a C++ library which uses pthreads to break down rendering tasks. This is for devices running Android 4+.

Lets say I have a 100 x 100 array of elements into which I repetitively do CPU-intensive processing. Currently I'm breaking the array up into four 25 x 100 element chunks and handing it off to four Posix threads (from a pool of stalled, pre-created threads). This gives an almost 4x speed increase on iOS and desktop Mac but slower results than single-threading under Android.

So the same code is used successfully to speed up the app on iOS or desktop Mac but in Android it often makes it even slower. I have done some tests on it and only quite big junks of data speed up when using multi threading. If the whole process (all threads) takes around 2 seconds or more it will speed up in multi threading mode but if it is less (say only takes about 400ms) it will be either the same speed or slower than just calling the rendering function normally. Which could point to thread switching being really slow. The bigger the processing tasks, the more they profit from multithreading. My tasks are usually not as big, but not fast enough in single threading mode.

I have also noticed that on ARM builds the speed difference between slower multi threading and the faster single threading is quite significant (almost twice as fast in multi threading rather than single threading) whereas on x86 builds the multi and single threaded versions will run at about the same speed as single threading on ARM builds. So x86 builds do not get slower on multithreading but also not faster.