### 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.
_hosts=None, _hosts_all=None, stage='task'):
        '''
        Returns the variables, with optional "context" given via the parameters
        for the play, host, and task (which could possibly result in different
        sets of variables being returned due to the additional context).
        The order of precedence is:
        - play->roles->get_default_vars (if there is a play context)
        - group_vars_files[host] (if there is a host context)
        - host_vars_files[host] (if there is a host context)
        - host->get_vars (if there is a host context)
        - fact_cache[host] (if there is a host context)
        - play vars (if there is a play context)
        - play vars_files (if there's no host context, ignore
          file names that cannot be templated)
        - task->get_vars (if there is a task context)
        - vars_cache[host] (if there is a host context)
        - extra vars
        ``_hosts`` and ``_hosts_all`` should be considered private args, with only internal trusted callers relying
        on the functionality they provide. These arguments may be removed at a later date without a deprecation
        period and without warning.
        '''
        display.debug("in VariableManager get_vars()")
        all_vars = dict()
        magic_variables = self._get_magic_variables(
            play=play,
            host=host,
            task=task,
            include_hostvars=include_hostvars,
            include_delegate_to=include_delegate_to,
            _hosts=_hosts,