### 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.
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type
import itertools
import operator
from copy import copy as shallowcopy
from functools import partial
from jinja2.exceptions import UndefinedError
from ansible import constants as C
from ansible import context
from ansible.errors import AnsibleError
from ansible.module_utils.six import iteritems, string_types, with_metaclass
from ansible.module_utils.parsing.convert_bool import boolean
from ansible.errors import AnsibleParserError, AnsibleUndefinedVariable, AnsibleAssertionError
from ansible.module_utils._text import to_text, to_native
from ansible.playbook.attribute import Attribute, FieldAttribute
from ansible.parsing.dataloader import DataLoader
from ansible.utils.display import Display
from ansible.utils.sentinel import Sentinel
from ansible.utils.vars import combine_vars, isidentifier, get_unique_id
display = Display()
def _generic_g(prop_name, self):
    try:
        value = self._attributes[prop_name]
    except KeyError:
        raise AttributeError("'%s' object has no attribute '%s'" % (self.__class__.__name__, prop_name))
    if value is Sentinel:
        value = self._attr_defaults[prop_name]
    return value
def _generic_g_method(prop_name, self):