OP, good luck - I understand completely about wanting to be faster to complete a century.
In my area centuries are offered on only a few rides and there are enforced cut-offs where the route is closed and no support after a certain time.
Route organizers have overhead costs to provide SAG, security and medical professionals, venue rentals - money that is diverted from benefiting their charitable cause the longer the event runs.

The Wichita Falls "Hotter 'n Hell Hundred" diverts riders from the century route if you do not make "Hell's Gate" by their cutoff time.
The ride begins at sunrise. Last year they allowed an option to start in twilight to allow cyclists a better chance to finish before an early cutoff.
Most riders do not carry sufficient fluids/food to finish this ride unsupported and even if they can, it's still better to finish before afternoon heat sets in.
Many people finish this ride in the medical tents and SAG wagons due to dehydration/electrolyte imbalances/heat exhaustion.
The cutoffs are to encourage riders to finish safe and healthy and not attempt something they haven't prepared for.