Sure it is. Impostor syndrome is defined as the state of affairs where your emotional assessment of your competence is lower than your actual competence. The definition does not depend on which aspect of the task is being discussed. "I can't write code" (when you can) and "I'm too slow" (when the people who appear faster, are doing it by just sweeping bugs under the carpet) are both impostor syndrome.
I will add that by the sound of things, the reason you have impostor syndrome is because your previous bosses have been bullies, sadists and sociopaths who successfully accomplished their goal of damaging your morale for their own pleasure. This is sadly not a rare state of affairs, but it's not universal either. Never be reluctant to look for another job if your boss is an asshole.