Flink allows to read from a Kafka topic, is that a performance bottleneck making Flink slower overall?
Kafka partitions can scale horizontally to accomodate for higher thoughput.
One Flink consumer thread can only be assigned to one Kafka partition.
So, if you have only 1 Kafka partition, and N+1 Flink executors, then you will have N idle tasks, which could be a bottleneck, sure, but that is a tradeoff of having total-ordering within a Kafka topic, not necessarily a Flink problem.
Otherwise, you would create your Kafka topics with ten to hundreds of partitions, and Flink would be fine to consume it.