### Summary

Executing a playbook with many hosts, there is massive overhead in checking whether the hosts list is a pattern, see for yourself on this callgrind graph:

![grafik](https://user-images.githubusercontent.com/3154871/111359016-4a4bf480-868b-11eb-8c8f-55b6a6a10bd7.png)

https://github.com/ansible/ansible/blob/da60525610a384bb04833b1c6429d9db6a87ef64/lib/ansible/vars/manager.py#L490

That check has quadratic runtime with the number of hosts - with a lot of hosts, commenting that check out reduces runtime manyfold.

### Issue Type

Bug Report

### Component Name

variable manager

### Ansible Version

2.10.5, devel

### OS / Environment

Ubuntu

### Steps to Reproduce

```yaml
- hosts: localhost
  tasks:
    - set_fact:
        extra_tasks: true # compute it
        hosts: "{{ [an, array, with, 5100, computed, hosts] }}"

- hosts: "{{ hostvars.localhost.hosts }}"
  vars:
    extra_tasks: "{{ hostvars.localhost.do_extra_tasks }}"
  tasks:
   # some more tasks here
    - name: Execute extra tasks
      command: /bin/true # example
      when: extra_tasks
```

### Expected Results

Execute in tens of seconds.

### Actual Results

Take over 5 minutes to execute.
if target_host in self._active_connections:
                connection = Connection(self._active_connections[target_host])
                del self._active_connections[target_host]
            else:
                connection = plugin_loader.connection_loader.get(play_context.connection, play_context, os.devnull)
                connection.set_options(task_keys=task.dump_attrs(), var_options=all_vars)
                play_context.set_attributes_from_plugin(connection)
            if connection:
                try:
                    connection.reset()
                    msg = 'reset connection'
                except ConnectionError as e:
                    
                    display.debug("got an error while closing persistent connection: %s" % e)
            else:
                msg = 'no connection, nothing to reset'
        else:
            raise AnsibleError("invalid meta action requested: %s" % meta_action, obj=task._ds)
        result = {'msg': msg}
        if skipped:
            result['skipped'] = True
            result['skip_reason'] = skip_reason
        else:
            result['changed'] = False
        display.vv("META: %s" % msg)
        res = TaskResult(target_host, task, result)
        if skipped:
            self._tqm.send_callback('v2_runner_on_skipped', res)
        return [res]
    def get_hosts_left(self, iterator):