### 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.
''' returns list of available hosts for this iterator by filtering out unreachables '''
        hosts_left = []
        for host in self._hosts_cache:
            if host not in self._tqm._unreachable_hosts:
                try:
                    hosts_left.append(self._inventory.hosts[host])
                except KeyError:
                    hosts_left.append(self._inventory.get_host(host))
        return hosts_left
    def update_active_connections(self, results):
        ''' updates the current active persistent connections '''
        for r in results:
            if 'args' in r._task_fields:
                socket_path = r._task_fields['args'].get('_ansible_socket')
                if socket_path:
                    if r._host not in self._active_connections:
                        self._active_connections[r._host] = socket_path
class NextAction(object):
    """ The next action after an interpreter's exit. """
    REDO = 1
    CONTINUE = 2
    EXIT = 3
    def __init__(self, result=EXIT):
        self.result = result
class Debugger(cmd.Cmd):