A bottleneck is the part of any process that limits the total process.
You cannot drink a beverage faster than it can come out of the neck of a bottle, no matter how big the overall bottle may be.
The bottleneck can be anywhere along of a process from start to finish. In chemistry are called "rate-limiting steps". In project planning it's called the "critical path".
Spending time working on the bottleneck helps improve your overall process speed. Time spent speeding up the other processes is wasted effort if you don't improve the bottleneck.
Do you know what the bottlenecks in your process are?