Generally speaking, it's not on purpose. There may be slight drops—you know, 5% or 10%—due to a new technology or a new way to work around things. A perfect example of this would actually be the Spectre and Meltdown fixes that have come out recently to address security flaws, where there is the potential for having your device run slightly more slowly as a result of these fixes.
On March 1st, 2018. See this statement in context.