AWS nat instance and elb bottle neck

Got a quick question which might be totally silly but it's early in the morning. So I have a standard AWS VPC with an ELB, NAT instance and 2x EC2 instances for the application code. By the looks of the image bellow the internet gateway passes traffic the ELB and NAT instance. My question is why isn't the NAT instance in front or behind the ELB ? it looks like that the single bottleneck in this VPC could be the NAT instance if all traffic is going through there.