If the employers, after an investigation, believe the behaviour is due to capability, they will, more often than not, put someone on a Performance Improvement Plan (PIP). Employers have a duty of care to ensure employees are suited for their roles and that their health is not put at risk.
If your employer has not complained about your performance before you submit a grievance, it is often the case that employers 'suddenly' notice your lack of capability after you have complained about something. This can be emotionally stressful and impact your actual performance. Luckily, you can take some steps to protect against some of the backlashes you might face if you complain about something in the workplace.
By taking a small precaution before submitting your grievance you can render this action by the employer much less damaging. Essentially what you should do is to highlight how any such potential action by the employer might be seen as victimisation or detrimental treatment and a breach of employment legislation as part of your letter.
The harassment/victimisation/discrimination experienced is having a significantly negative impact on my ability to perform my role. An omission to act on my grievance will demonstrate bad faith and is a serious breach of the good faith performance.
Obviously, edit the harassment/victimisation/discrimination bit to fit the topic of your grievance.