It is extremely important to note that publishing arguably the most ground breaking mathematical paper of all time is not a quick process, especially given that the realm of mathematics explored in The Principia Mathematica had never been touched on before, so exploring everything - which he essentially did - took a great deal of time. What we learn about Calculus in high school barely scratches the surface of what he actually did. Newton was also a man of many side projects. In 1666 there was a fire in London, and an outbreak of plague drove Newton to retire to his mother's home in Woolsthorpe. This is where Newton conducted prism experiments, discovering the spectrum of light; works out his system of "fluxions," the precursor of modern calculus; and began to consider the idea of gravity. Newton continued with these studies semi-concurrently for most of the next 3 decades of his life.
For reference, Newton sent his paper on optics and prism experiments to the Royal Society in 1672, however, the book regarding this field (Opticks) was published 32 years later. In large part, the 32 years was the result of unconventional, less concrete proof methods (refer to i for the technicalities). These methods entailed stern speculation and controversy among scientists (including Hooke), all of which were/still are expected when validating such an unprecedented idea. On top of this, Newton was developing Calculus which occupied a large amount of his time during the process.
The development of Calculus really began in the early 1670's after Newton really grasped the ideas of optics to the level where materializing his results was possible. Throughout that decade, Newtons spent his time exploring the ideas of Calculus trying to numerically piece together observations, knowns, and at the time, theoretical mathematical properties. This took years, finally finishing his calculations in November of 1684. Over the next 3 years, the ideas Newton put forward were analyzed for validity, and eventually published in 1687. 10-14 years to mathematically prove groundbreaking concepts and have them published for the world to see is not unreasonable at all, if anything, it is remarkably fast.
Note: the story behind this is too long to fully explore, so I simplified it by leaving out Leibniz, Halley, and the other individuals that played a large role. Also, I simplified what I said about the Principia to only regard Calculus, which is the most relevant. The Principia is 450+ pages long, so discussing how each aspect of it delayed the publication date would be a novel in and of itself.
TLDR; Publishing a groundbreaking mathematical paper takes time - a lot of time, and working on numerous unprecedented works at once does not speed up the process.
i: Unlike the Principia, Opticks is not developed using the geometric convention of propositions proved by deduction from either previous propositions, lemmas or first principles (or axioms). Instead, axioms define the meaning of technical terms or fundamental properties of matter and light, and the stated propositions are demonstrated by means of specific, carefully described experiments.
Sources:
http://assets.cambridge.org/97805210/45858/frontmatter/9780521045858_frontmatter.pdf
https://school.bighistoryproject.com/media/khan/articles/U2_Issac_Newton_2014_780L.pdf