### 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 ansible.module_utils._text import to_text
from ansible.module_utils.common._collections_compat import Mapping, MutableMapping, Sequence
from ansible.module_utils.six import iteritems, text_type, string_types
from ansible.plugins.loader import lookup_loader
from ansible.vars.fact_cache import FactCache
from ansible.template import Templar
from ansible.utils.display import Display
from ansible.utils.listify import listify_lookup_plugin_terms
from ansible.utils.vars import combine_vars, load_extra_vars, load_options_vars
from ansible.utils.unsafe_proxy import wrap_var
from ansible.vars.clean import namespace_facts, clean_facts
from ansible.vars.plugins import get_vars_from_inventory_sources, get_vars_from_path
display = Display()
def preprocess_vars(a):
    '''
    Ensures that vars contained in the parameter passed in are
    returned as a list of dictionaries, to ensure for instance
    that vars loaded from a file conform to an expected state.
    '''
    if a is None:
        return None
    elif not isinstance(a, list):
        data = [a]
    else:
        data = a
    for item in data:
        if not isinstance(item, MutableMapping):
            raise AnsibleError("variable files must contain either a dictionary of variables, or a list of dictionaries. Got: %s (%s)" % (a, type(a)))
    return data
class VariableManager: