Could this be Daniel F. Galouye's The Infinite Man (1973)? Per the Wikipedia summary:
A research project, Project Genesis, searches for evidence confirming
the Steady State Theory of continuous creation in the area within and
surrounding an unnamed American Midwestern city, according to which,
125,000 newly created neutrons (also called neoneutrons, which decay
into hydrogen atoms consisting of protons and electrons while giving
off hard gamma radiation - here called creation-radiation) should be
called into existence in an Earth-sized volume in a twenty-four-hour
period. The monitored area is an equilateral triangle with thickness
of 50 feet (15 m) and an area of 300 square miles (780 km2) which
should, by a proportional estimate, register twenty-one neoneutronic
creation signatures in that same period.
Suddenly, not merely twenty-one, but millions of neoneutrons are
called into existence in this monitoring area, fusing some of the
sensor equipment. After replacing and refusing of the sensor elements,
the locus of the phenomenon is traced to a rail-yard on the edge of
the zone, and further, apparently emanating from a young drug-user and
hobo, Milton Bradford.
Five years later, Milton Bradford (or, "Brad" to his inner circle) has
gone from drug-using poverty to the pinnacle of corporate power as the
Chairman of Progress and Development Enterprises (P&D), a real-estate
and industrial conglomerate who functioned as an important partner
during the days of the Genesis Project...
The reality is that P&D Enterprises, while a real company, is solely
devoted to providing a safe and contented existence for Bradford. For,
earlier on, psychological probing has revealed to Project Genesis'
staff that the phenomenon that led them to Bradford is in fact the
Creative Force that brought the Universe into being in the first
place, and (while not immediately clear to the Project staff) the
Creative Force has sought Bradford as a hiding place from a Universe
that has become tiresomely overwhelmingly complex and the Destructive
Force whom the Creative Force had originally brought into being a
self-made opponent to stave off ennui. The Creative Force now seeks
shelter amongst its favored beings.
The Wikipedia summary also mentions the specific detail that you recall regarding the change in the speed of light:
Between the adoration of the cult, which its leader has increasing
trouble restraining, and the staff of P&D, psychologically exhausted
from years of maintaining the P&D fiction against the utter
destruction of the Universe, the cracks in the cover story develop and
widen, and the Creative Force arouses and begins to try to simplify
the Universe in order to make it more manageable - though not
uninhabitable for its chosen residents; from the original confirmatory
nova of Proxima Centauri and destruction of Pluto, it deletes all
quasars observable in the Universe; rationalizes pi at the 323rd
decimal; halves the speed of light; and changes probability so that
outcomes bracketing the mean become more likely than the mean itself.
These revisions to physical reality typically happen after dreams; the
quasars are seen in Bradford's dreams as "glowflies", as an example;
and the halving of the speed of light is expressed as "half a sea".
If not the novel, you may be thinking of one of the novellas from which the novel was formed. Additional details of the novella versions can be seen in this previous answer.