Adding more yeast to fermenting homebrew beer will not speed up the process of fermentation. However, using yeast starters and ensuring the wort is at an ideal temperature (around 68 °F) when pitching can help fermentation get started more quickly, reducing total time in the fermenter.
In order to speed up the beginning stages of the fermentation process, you can make what’s called a yeast starter. A yeast starter is simple to make. By boiling water and malt extract, waiting for it to cool, and combining it with liquid yeast, you will have made your yeast starter.
During the fermentation process, the yeast is converting the sugar into alcohol, as we have discussed. So in theory, if there is not enough sugar for the yeast to convert into alcohol, your ABV will not be higher.
Speeding up the fermentation process should be done with caution, and an understanding of everything that could go wrong.
The purpose of making a yeast starter is to allow the yeast organisms to multiply. This takes the strain off of the yeast that is already working, the more live yeast organisms, the more help they get fermenting.
High temperatures will make beer ferment quicker, and conversely low temperatures will cause it to ferment slower.
A 3-gallon batch vs a 10-gallon batch will take about the same time to ferment. However, if you need to adjust the temperature at all, 3-gallon batch will heat up (or cool down) more quickly than a 10-gallon batch.
The lower the ABV, the quicker the fermentation process. Lower ABV means less work for the yeast, and therefore a quicker turnaround.
Kegging your beer can shave a few days off of your fermentation time. When bottle conditioning, the yeast needs at least a few weeks to convert the priming sugar from the bottles to alcohol.
There are many forces at play here, and manipulating any of them even in the slightest can speed up (or slow down) the process of fermentation. Using a yeast starter can jump-start the process for you, though!