### 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.
''' merges all entities by inventory source '''
                return get_vars_from_inventory_sources(self._loader, self._inventory._sources, entities, stage)
            def _plugins_play(entities):
                ''' merges all entities adjacent to play '''
                data = {}
                for path in basedirs:
                    data = _combine_and_track(data, get_vars_from_path(self._loader, path, entities, stage), "path '%s'" % path)
                return data
            
            def all_inventory():
                return all_group.get_vars()
            def all_plugins_inventory():
                return _plugins_inventory([all_group])
            def all_plugins_play():
                return _plugins_play([all_group])
            def groups_inventory():
                ''' gets group vars from inventory '''
                return get_group_vars(host_groups)
            def groups_plugins_inventory():