but kafka isn't fast. Most things are backed by real files, so when you hit limits or something ejected from cache, it gets slow real fast.

Kafka isn't the right choice for most things.

SQS, MQTT, NATS, rabbit if you're wanting a lot of admin are all better (plus the crap that azure and google make)

kafka is not for latency, it is used for high throughput. By design kafka shines in high throughput workload due to consumer and producer concurrency (consumer group), broker concurrency (multiple nodes and partitions).

for latency sensitive you will probably need redis pub/sub or something in-memory

In all the times I've been forced to use Kafka, I have never seen single digit millisecond latency.

If you need fast response Kafka is a bad choice.

If you are okay going to multiple digits of milliseconds then there are simpler solutions.

The only reason to use Kafka is the ability to guarantee order. For everything else it's second place at best.