For much of the time photography has been around ‘film speeds’ have been slow, needing quite a lot of light to record an image. The reason the Victorians stuck with wet-plate photography as long as they did, was because it was the only way of getting enough sensitivity out of a plate to record a living, breathing and moving subject.
Even after dry plates and films came on the scene, the evolution of sophisticated fast emulsions was slow in coming, and ways were always being sought to squeeze more performance out of a film. Even now, when film technology can be considered almost perfected, there are occasions where a bit more speed might be useful.
The film has its own inherent speed, hopefully around what the manufacturer puts on the film box as an ISO number, But the first thing to realise is that film speed is in the eye of the beholder, and film manufacturers tend tends to eye their products rather benignly. Forcing, or pushing, by extending development time will tend to make underexposed details visible, but grain will tend to clump and become more harsh, while contrast increases and highlights block up.
Most developers can be used to ‘cook’ a film to some extent, some more successfully than others. The highly active acutance developers designed to be used very dilute, such as Neofin Blue and Rodinal will give full film speed, but by their nature don’t hold the reserve needed to boost density to a marked degree.
So before coupling a film with a developer decide what the aim is and what sort of treatment might be required. We’ve take the liberty of dividing what are normally regarded as speed-increasing developers into 2 camps, those that give good shadow detail, (which an be interpreted as a moderate speed increase) which we’ve called ‘Speed Revealing’. Those designed to put film through the wringer are labelled ‘Speed Forcing’.
But above all bear in mind that it’s not really possible to fundamentally alter the film speed, one is simply adjusting the way the film copes with what you’re giving it. So by uprating film one is under-exposing it, then compensating by making the developed result more printable.