Delay between onStart and first onProgress

[YasserMojahed]

I am using ffmpeg in my app to split and merge some videos. It is fast if I just started my app and it goes to onProgress at the same second of executing the command.
However, After Execute many commands it becomes slower and there will be a big delay between onStart and first onPrpgress (sometimes 10 minutes!!) but there is no delay in progress itself.
I think there is something holding on the command. how can I fix this?