I suspect that the kernel prioritize getting fsync done above all else. This because data written to disk is data safe from sudden power loss. This means that a

I suspect that the kernel prioritize getting fsync done above all else. This because data written to disk is data safe from sudden power loss. This means that all other calls, ui draw calls included, gets put aside until the fsync is done. And that depends on the IO speed of the hardware, and the amount of data in the queue. On older android devices i have run apps that could show IO rates. And invariably when things would lag or freeze the IO would be maxed out (one offender would be Facebook and their Messenger, because after each update they would force a recompile or something).