<p>The e3D volcano features an extended heater block of length 20&nbsp;mm with the cartridge heater running parallel to the filament.</p>

<p>The purpose of this is to increase the speeds at which filament can be printed (of course the extruder and other factors may still be limiting factors).</p>

<p>My question is how capable would this heat block be of printing at slow speeds with a 0.4&nbsp;mm nozzle?</p>

<p>Is printing still possible at lower speeds or is the filament heated too much that jams occur? Is the retraction performance okay?</p>

Speaking from first hand experience running a Volcano hotend, mostly using a 0.6 mm nozzle, but I have used 0.4 mm as well. I can't really complain about any lower printing speed limit (low speeds are usually a solution to high speeds problems for me).
Just for completeness: I am using a DaVinci 1.1 Plus with custom firmware (modified Marlin) and an E3D Volcano hotend. No problems with the nozzle, clean prints! Note: I am using the Titan Aero as extruder but not using the included pancake motor! Went with the original motor of the DaVinci.
And as always: the parameters are key! Given a bit of tuning you can get amazing results! For PLA and ABS I can work without stringing. Although ABS seems to be prone to pitting (slight underextrusion at start of path). TPU and other flex are sometimes a bit of a challenge, but that's mainly due to my own lack of experience there.
The one important caveat here would be to also reduce the nozzle temperature. A possible theory here might be that the filament has more time and surface to heat up.
(Sometimes I go as much as 15 °C lower as compared to normal/high speeds! Usually lower speeds means small pieces for me and in term means a limit to the layer cooling time.)