Possible performance regression with pg_dump of a large number of relations

I've been troubleshooting an issue with slow pg_dump times on postgres 9.6.6. I believe something changed between 9.5.10 and 9.6.6 that has made dumps significantly slower for databases with a large number of relations. I posted this in irc and someone suggested that I should post this here. I'm sorry if this isn't the right place.

To simulate the issue I generated 150,000 relations spread across 1000 schemas (this roughly reflects my production setup).

```ruby
File.write "many_relations.sql", (150000 / 150).times.flat_map {|n|
[
"create schema s_#{n};",
150.times.map do |t|
"create table s_#{n}.test_#{t} (id int);"
end
]
}.join("\n")
```

I have 2 identical pieces of hardware. I've installed 9.5 on one and 9.6 on the other. I've run the same generated piece of sql in a fresh database on both systems.

On my 9.5.10 system:
> time pg_dump -n s_10 testing > /dev/null
real 0m5.492s
user 0m1.424s
sys 0m0.184s

On my 9.6.6 system:
> time pg_dump -n s_10 testing > /dev/null
real 0m27.342s
user 0m1.748s
sys 0m0.248s

If I call that same pg_dump command with the verbose option, the delay is at `pg_dump: reading user-defined tables` step.

I don't have identical hardware, so I can't say for sure, but I believe this issue is still present in 10.1.

Is this a legitimate issue? Is there more information I can provide to help better assess the situation?

Thanks in advance everyone!

Luke