Didn't want your post to go unanswered but just doing breakfast for dd so maybe a haphazard post 
I would prepare yourself for the meeting by speaking to your line manager about it before hand to ask the purpose of it. I would also request a copy of your job description too, and review what you are doing within that description and what isn't listed (ie menial tasks).
Whilst the meeting sounds scary as HR are being invited in, this is an opportunity for you to say how you feel about what is happening and the comments your boss has been making. If menial tasks are being completed by you, and you are senior management- is that value for money for the organisation? Probably not. Your time and expertise could be put somewhere else.
It could be that, over time your role has altered and they need you to take on other things (the menial tasks) but haven't communicated this properly to you. It may be that they are not aware you are working additional hours outside of your contract.
Ultimately, they see a problem (whether that is performance issue or not, it is unclear) and the meeting should be about solutions.
If it is a performance meeting, it sounds fairly informal at this stage (even with HR there) as it isn't a disciplinary meeting (you would have been formally invited in, something they'd struggle to do if you have never had a meeting prior to it to notify you of any performance issues).
When is the meeting?