userspace process
                                        ^              |
                                        |              |
                                   _____|____     ____\/___
                                  /          \   /         \
                                  |   input   |  |  output  |
                                  \__________/   \_________/
                                       ^               |
                                       |               |
    _________      __________      ---------     _____\/_____
   /         \    /          \     |Routing |   /            \
-->  ingress  ---> prerouting ---> |decision|   | postrouting |--> neigh_xmit
   \_________/    \__________/     ----------   \____________/          ^
     |      ^                          |               ^                |
 flowtable  |                     ____\/___            |                |
     |      |                    /         \           |                |
  __\/___   |                    | forward |------------                |
  |-----|   |                    \_________/                            |
  |-----|   |                 'flow offload' rule                       |
  |-----|   |                   adds entry to                           |
  |_____|   |                     flowtable                             |
     |      |                                                           |
    / \     |                                                           |
   /hit\_no_|                                                           |
   \ ? /                                                                |
    \ /                                                                 |
     |__yes_________________fastpath bypass ____________________________|

table inet x {
        flowtable f {
                hook ingress priority 0; devices = { eth0, eth1 };
        }
        chain y {
                type filter hook forward priority 0; policy accept;
                ip protocol tcp flow offload @f
                counter packets 0 bytes 0
        }
}

[email protected]:~$ cat nat.nft 
flush ruleset

table ip filter {
	flowtable fastnat {
		hook ingress priority filter
		devices = { eth0, eth1 }
	}

	chain forward {
		type filter hook forward priority filter; policy accept;
		ip protocol { tcp, udp } flow add @fastnat
	}
}
table ip nat {
	chain POSTROUTING {
		type nat hook postrouting priority srcnat; policy accept;
		ip saddr 192.0.2.0/24 oif "eth0" snat to 192.168.122.14 persistent
	}

	chain PREROUTING {
		type nat hook prerouting priority dstnat; policy accept;
	}
}