Developers are often slow (if they really are in this case) when they are working on something for long, lost focus, not working on something that is exciting or not sure how working fast will be beneficial for them.

What works for me is to sit with them, understand their problems (in technical or not technical ways), document the work in actionable forms, have smaller goals (deadlines) and continue checking progress (I repeat same thing if required - just make sure that I am still fun).

I lead a team of 100 people and have been doing this for 13 years. I understood that things work well when I get involved (not micromanaging) and solve problems together.