Any peripheral and pointless device which is added to a product to make it appear to perform better.
This term began in the UK in the 1970s with 'go-faster stripes' - the decorative stripes which were added to the bonnets or sides of cars to make them appear more sporty; for example, this item from The Times, April 1971 Times:
"Three exclusive colours are specified, with some strikingly extrovert ‘go-faster’ stripes if you want them."
The stripes were intended to ape the appearance of sports cars which were modified for racing and which were often painted with designs in team colours. 'Go-faster' has since been used in other contexts as a derisive comment on any ineffectual attempt to pass off a product as more powerful than it really is.