I have a duel flush toilet cistern when after flushing takes a long time to refill, how do I turn up the water flow?

Thanks

Are you 100% sure the shut off under the toilet is fully opened? If opened it may have some debris and can be blown out after disconnecting the toilet fill line.

You may have some debris in the fill valve. Turn water off to the toilet, remove the fill valve, clean its water passages and reinstall. Turn water back on.

All are good suggestions, I think its logical to try them in the order we have listed them.