You must meen 16th notes. They are 4 notes per beat. And eighth notes, they are two notes a beat.
Anyway, I'd say try to limit your right hand movement, and keep the movement in your thumb and index finger. Also, perhaps the reason you lost your speed was just lack of practise, if even for a few weeks. I once could play 16ths at around 155 bpm, but then I stopped practising that. Then when I resumed, I was only around 130. I had to work my way up from 130 back to where I was before.