Time machine backup very slow with SMB!

I am pretty new to NAS, just got my Synology 923+ NAS and I am doing time machine backup for Macbook pro to NAS. I am only getting 500 kB/s 1000 kB/s. I am doing it via Wi-Fi still the speed I am getting is very less than what it should be. When I run iperf3 I get 25-30 mB/s even in Wi-Fi. MacOS is BigSur 11.7.5 and my hard disks are 2 8TB Ironwolfe 7200rpm with SHR config. Attached are the speed data. Any of you have experienced similar issue?