### 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.
''' validation that is done at parse time, not load time '''
        all_vars = {} if all_vars is None else all_vars
        if not self._validated:
            
            for (name, attribute) in iteritems(self._valid_attrs):
                if name in self._alias_attrs:
                    name = self._alias_attrs[name]
                
                method = getattr(self, '_validate_%s' % name, None)
                if method:
                    method(attribute, name, getattr(self, name))
                else:
                    
                    value = self._attributes[name]
                    if value is not None:
                        if attribute.isa == 'string' and isinstance(value, (list, dict)):
                            raise AnsibleParserError(
                                "The field '%s' is supposed to be a string type,"
                                " however the incoming data structure is a %s" % (name, type(value)), obj=self.get_ds()
                            )
        self._validated = True
    def squash(self):