Why is ffmpeg dropping frames?
The issue is that the pixel format within each input changes mid-way. This causes the filtergraph to reinitialize and buffered frames get dropped. Suppress reinitialization and convert to a command pixel format before blending.
What is FPS in ffmpeg?
ffmpeg -r value (fps) = ave speed (m/s) / frame spacing (metres) e.g. for spacing every 5 metres at average speed of 8.33 m/s.