Configure NAS to use memory cache for all available capacity when copying files over SMB

I have Synology DS923+ NAS with 32 GB memory and two hard drives in RAID1. It runs Linux 4.4 with the ext4 filesystem. Copying a 20GB file from my laptop to NAS using SMB with 10GbE wired ethernet the transfer is 500MB/s, but after 10%, slower than 100MB/s (same as direct HDD write speed). I changed to vm.dirty_ratio from 30% to 90%. It's ok. It use 4% when normal idle. and changed to vm.dirty_background_ratio from 10% to 3% for faster cache flushing. And... It sends more than 500MB/s to 90%. but last 10% was slowing down: What else can I do to make sure that even in the last 10%, I'm still getting the maximum transfer speed of the network?