The gap between two channels is not closed by speed. It is closed by having one pool of availability that every channel reads from, instead of several copies that each believe themselves.
Almost no double booking is caused by a slow system. It is caused by the same room being recorded in several places at once, with each place trusting its own copy. As long as the truth exists in more than one version, there is always a moment where two versions disagree — and speed only makes that moment shorter, never absent.
Picture the last room for a Saturday night, listed on two channels. Two guests book seconds apart. Each channel checks its own copy of availability, sees the room as free, and accepts. By the time the close-out instruction reaches the second channel, the second reservation already exists.
Nobody made a mistake. Both channels behaved exactly as designed. The failure is architectural: two systems were each allowed to believe they owned the same room.
Linking calendars with iCal is the most common first step, and it is a reasonable one. But iCal is a pull-based format: the other side fetches your calendar file on its own schedule, typically measured in tens of minutes, and sometimes longer when the platform is busy. Nothing about it is instant, and nothing about it is guaranteed.
Moving from iCal to a real connection shortens the window dramatically, from minutes to seconds. That is a genuine improvement and it removes most incidents. But it does not remove the category, because a push-based design still means several copies exist and are being reconciled after the fact.
There is also a subtler failure that speed cannot touch. Some channels apply their own automatic adjustments to availability — adding a room back when a cancellation arrives, for example. If your system writes an absolute number at the same instant the channel is applying its own delta, the two operations can land in either order, and the result is sometimes higher than the truth.
The structural fix is to stop keeping copies. Availability is computed from one pool of real rooms, and every channel is a view onto that pool rather than an owner of part of it. When a reservation arrives from anywhere, the pool shrinks, and every channel is corrected to match the new count.
That last point matters more than it looks. A system that cannot read the current state of a channel has two options: guess, or say so. Guessing in the direction of “probably still available” is exactly how a room gets sold twice. Refusing to guess is what makes the difference between a system that is usually right and one you can actually rely on during a full house.