### 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.
if omit_value is not None and value == omit_value:
                    if callable(attribute.default):
                        setattr(self, name, attribute.default())
                    else:
                        setattr(self, name, attribute.default)
                    continue
                
                if value is not None:
                    value = self.get_validated_value(name, attribute, value, templar)
                
                setattr(self, name, value)
            except (TypeError, ValueError) as e:
                value = getattr(self, name)
                raise AnsibleParserError("the field '%s' has an invalid value (%s), and could not be converted to an %s."
                                         "The error was: %s" % (name, value, attribute.isa, e), obj=self.get_ds(), orig_exc=e)
            except (AnsibleUndefinedVariable, UndefinedError) as e:
                if templar._fail_on_undefined_errors and name != 'name':
                    if name == 'args':
                        msg = "The task includes an option with an undefined variable. The error was: %s" % (to_native(e))
                    else:
                        msg = "The field '%s' has an invalid value, which includes an undefined variable. The error was: %s" % (name, to_native(e))
                    raise AnsibleParserError(msg, obj=self.get_ds(), orig_exc=e)
        self._finalized = True
    def _load_vars(self, attr, ds):
        '''
        Vars in a play can be specified either as a dictionary directly, or
        as a list of dictionaries. If the later, this method will turn the
        list into a single dictionary.
        '''
        def _validate_variable_keys(ds):