First, note: this answer may only apply to certain regions where it is illegal to dismiss an employee without severance. That said...
This could be a case of Constructive dismissal and which is illegal.
The tactic is to demoralize and lower the self-esteem of an employee until they quit the job. It's a way for the company to save money by not having to pay severance, or solve the problem of having to confront the employee and fire them.
He wants to talk to me about why I am so slow and why my bug fix rate is so low.
This fault is very ambiguous. It's impossible for either side of the party to claim the other is wrong. You took a month to fix one bug. So what! This places you in a defensive position, by having to present facts to support your claim that a month was required. Given your current skill, experience and knowledge as factors. As an employer it's the employer's job to manage the time and efforts of his employees. The employer must be the person engaging in the risk associated with having the bugs fixed. Not the employee. He always had the choice to assign the bug to someone else.
If you are a contractor, and it states in your contract that will be responsible for the fixing of bugs, then it's a completely different story.
Is it wrong for the employer to complain that you are taking too long? Absolutely not, but he can not hold you accountable for it, and he can not fire you for it. He can say to you "We have no more bugs that require your skills, and you are placed on leave," but they must tell you the moment a new issue comes up that you can fix. Otherwise, they must terminate with severance. What he can not do is give you work you can't handle and then complain about it. I think this is illegal.
I love programming and solving problems but I actually do find my job really really hard.
There is a big difference between taking a job you find hard, and your employer giving you work that is too hard. If you feel the tasks assigned to you were done to discourage you from having a career with the company, this could be illegal.
I've actually been a programmer for about 10 years. But this is my first multithreading embedded linux job - I've been here 2 years and it's obvious to everyone that I'm still struggling.
This is why I think you've found yourself in the middle of a constructive dismissal. They aren't happy with you so they pile the crap on you until you leave.
And I think I've become so demoralised and feel so marginalised that I've lost a lot of the fire that I had at the start of the job.
An employer is responsible for providing a safe and positive work environment. Without more information (most likely personal) it's difficult for outsiders to say what is really going on here. Ask an employment lawyer for a free consultation. They will be able to tell you if you are being played.
I'm not a lawyer, but did Google some documents discussing the topic of Constructive dismissal which are worth reading before you enter your review on Monday. The main point here is to watch out for a reduction in salary, humiliation and sudden changes to your career with the company.
Relevant facts watch out for:
- Failure to properly support an employee during difficult working conditions
- Excessive disciplining of an employee Imposing a change in the employees place of work at short notice
- Imposition of a reduction in salary or wages
Legal Q&A: Constructive dismissal
Reasons to claim for constructive dismissal
elements of a constructive dismissal