### 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.
try:
                facts = wrap_var(self._fact_cache.get(host.name, {}))
                all_vars.update(namespace_facts(facts))
                
                if C.INJECT_FACTS_AS_VARS:
                    all_vars = _combine_and_track(all_vars, wrap_var(clean_facts(facts)), "facts")
                else:
                    
                    all_vars = _combine_and_track(all_vars, wrap_var({'ansible_local': facts.get('ansible_local', {})}), "facts")
            except KeyError:
                pass
        if play:
            all_vars = _combine_and_track(all_vars, play.get_vars(), "play vars")
            vars_files = play.get_vars_files()
            try:
                for vars_file_item in vars_files:
                    
                    
                    temp_vars = combine_vars(all_vars, self._extra_vars)
                    temp_vars = combine_vars(temp_vars, magic_variables)
                    templar = Templar(loader=self._loader, variables=temp_vars)
                    
                    
                    
                    vars_file_list = vars_file_item
                    if not isinstance(vars_file_list, list):
                        vars_file_list = [vars_file_list]