### 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 value is None:
                value = set()
            elif not isinstance(value, (list, set)):
                if isinstance(value, string_types):
                    value = value.split(',')
                else:
                    
                    
                    value = [value]
            if not isinstance(value, set):
                value = set(value)
        elif attribute.isa == 'dict':
            if value is None:
                value = dict()
            elif not isinstance(value, dict):
                raise TypeError("%s is not a dictionary" % value)
        elif attribute.isa == 'class':
            if not isinstance(value, attribute.class_type):
                raise TypeError("%s is not a valid %s (got a %s instead)" % (name, attribute.class_type, type(value)))
            value.post_validate(templar=templar)
        return value
    def post_validate(self, templar):
        '''
        we can't tell that everything is of the right type until we have
        all the variables.  Run basic types (from isa) as well as
        any _post_validate_<foo> functions.
        '''
        
        omit_value = templar.available_variables.get('omit')
        for (name, attribute) in iteritems(self._valid_attrs):