What is the best length of the melting zone in the hotend?
Short melting zone melts material in small amount which is suitable for thin layers with small nozzles. In opposite long melting zone can heat big amount of filament which is needed for fast printing and thick layers. 

short melting zone


less amount of melted material
thin layer heights
quality printing with details
give more precise volume control with less flow/pressure lag and better retraction performance (thx @Ryan-Carlyle)
nozzles: 0.25-0.8mm (available for E3Dv6)


long melting zone


more amount of melted material
thick layer heights
fast printing with less details
nozzles: 0.4-1.2mm (available for E3D Volcano)


Example

Example of those hotends are E3Dv6 and E3D Volcano

E3Dv6



E3D Volcano with much longer melt zone then v6



Speed comparison video: https://www.youtube.com/watch?v=_PQjwvWoGpk

Speed comparison graph: