How to display a form in controller method
Thanks to Kevin's comment, I realised that I can just return the form AND still use the custom access check reference to a controller method:

mymodule.single_tab:
  path: '/node/{node}/edit/performance'
  defaults:
    _form: '\Drupal\add_performance\Form\myForm'
    _title: 'Add performance'
  requirements:
    _custom_access: '\Drupal\add_performance\Controller\mymoduleController::checkAccess'
  options: