Actually, NY and other states have made it a lot easier to fire teachers based on performance, regardless of tenure. Tenure only gives teachers the right to due process. The problem isn't with tenure - part of the problem is with lazy administrators and school boards who don't want to bother with due process.
Another part of the problem is measuring poor performance. How do you measure poor performance consistently and fairly? "The contrast is starkstudents' test scores jumped the equivalent of an extra year and a half of schooling in math..." "And while The Equity Project's eighth graders outperformed students citywide on state math exams in 2013, only 43 percent of them passed. " So, are these teachers bad because 57% of their students failed? Or are these teachers good because in one year, their students made far more than one year's worth of progress in mathematics?