### 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.
'''
        Helper function to add a new host to inventory based on a task result.
        '''
        changed = False
        if host_info:
            host_name = host_info.get('host_name')
            
            if host_name not in self._inventory.hosts:
                self._inventory.add_host(host_name, 'all')
                self._hosts_cache_all.append(host_name)
                changed = True
            new_host = self._inventory.hosts.get(host_name)
            
            new_host_vars = new_host.get_vars()
            new_host_combined_vars = combine_vars(new_host_vars, host_info.get('host_vars', dict()))
            if new_host_vars != new_host_combined_vars:
                new_host.vars = new_host_combined_vars
                changed = True