I think the performance of Swiftkey improved about half a year ago. But I think around the same time I started noticing a significant drop in Swiftkey's accuracy. It "feels" less accurate to me than it was, although I use it with 3 enabled languages at once and I imagine that also brings lower accuracy by default. Still, I think it has become quite a bit worse than before, and I worry they did that on purpose as a compromise to improve performance.