Using nginx to simulate slow response time for testing purposes

You can slow the speed of localhost (network) by adding delay.
Use ifconfig command to see network device: on localhost it may be lo and on LAN its eth0.

• to add delay use this command (adding 1000ms delay on lo network device)

  tc qdisc add dev lo root netem delay 1000ms

• to change delay use this one

  tc qdisc change dev lo root netem delay 1ms

• to see current delay

  tc qdisc show dev lo

• and to remove delay

  tc qdisc del dev lo root netem delay 1000ms

I've modified a nginx config to use limit_req_zone and limit_req to introduce delays. The following reduces the rate of service to 20 requests per second (rate=20r/s). I've set burst=1000 so that my application would not get 503 responses.

http {
    limit_req_zone $binary_remote_addr zone=one:10m rate=20r/s;
    [...]
    server {
        [...]
        location / {
            limit_req zone=one burst=1000;
            [...]
        }
    }
}

The documentation is here. I do not believe there is a way to specify a uniform delay using this method.

My earlier answer works but it is more adapted to a case where all requests need to be slowed down. I've since had to come up with a solution that would allow me to turn on the rate limit only on a case-by-case basis, and came up with the following configuration. Make sure to read the entire answer before you use this, because there are important nuances to know.

    location / {
        if (-f somewhere/sensible/LIMIT) {
           echo_sleep 1;
           # Yes, we need this here too.
           echo_exec /proxy$request_uri;
        }

        echo_exec /proxy$request_uri;
    }

    location /proxy/ {
        internal;
        # Ultimately, all this goes to a Django server.
        proxy_pass http://django/;
        proxy_set_header Host            $http_host;
        proxy_set_header X-Forwarded-For $remote_addr;
    }

Important note: the presence or absence of forward slashes in the various paths makes a difference. For instance, proxy_pass http://django, without a trailing slash, does not do the same thing as the line in the code above.

The principle of operation is simple. If the file somewhere/sensible/LIMIT exists, then requests that match location / are paused for one second before moving on. So in my test suite, when I want a network slowdown, I create the file, and when I want to remove the slowdown, I remove it. (And I have cleanup code that removes it between each test.) In theory I'd much prefer using variables for this than a file, but the problem is that variables are reinitialized with each request. So we cannot have a location block that would set a variable to turn the limit, and another to turn it off. (That's the first thing I tried, and it failed due to the lifetime of variables). It would probably be possible to use the Perl module or Lua to persist variables or fiddle with cookies, but I've decided not to go down these routes.

Important notes:

1. It is not a good idea to mix directives from the echo module (like echo_sleep and echo_exec) with the stock directives of nginx that result in the production of a response. I initially had echo_sleep together with proxy_pass and got bad results. That's why we have the location /proxy/ block that segregates the stock directives from the echo stuff. (See this issue for a similar conflict that was resolved by splitting a block.)

2. The two echo_exec directives, inside and outside the if, are necessary due to how if works.

3. The internal directive prevents clients from directly requesting /proxy/... URLs.