I had a manager tell me I was slow at one of my last jobs as well.
A few months later, I was having trouble convincing her to give me a promotion/raise. I knew I was doing the same amount of work as everyone else, if not more, and I was fixing a lot of bugs caused by developers 2 or 3 levels above me. Our 1 on 1 meetings consisted of me listing the reasons why I deserved a raise, and my manager telling me no, but never a good reason why. She even read me the guidelines for what the company looks for in engineers at each level, and I was doing all of it and more, but still no raise.
I ended up quitting (not just because of the pay), and taking my skills to an awesome new job where I was considered a top engineer, and I'm really proud of everything I got done there. The new company gave me a raise when I asked for it because they valued me more than my previous company.
My personal advice is to look for a new job where you can apply your skills where the company needs them, and make a good impression from the beginning. The added bonus being that you'll be able to negotiate a higher salary with the new job as well.