There is definitely a performance ceiling if you use the RDMBS as your event store. I used to worry about that. Now that I've got a few years working at a very successful high frequency trading firm that did message queues in an RDBMS behind me, I don't worry about it so much anymore.
They didn't use it for everything; there was also a blindingly efficient homegrown messaging system for the stuff that needed to be really fast. But that was for fairly well-defined use cases. (Latency and concurrency were the key limiting factors.) By default, everything went into the RDBMS-based system. It was inherently more robust, it replaced a whole quagmire of design questions you have to make when using other messaging systems with ACID guarantees, and it made it a lot quicker & easier to do post-mortem analysis of any surprising things that happened in production.
They didn't use it for everything; there was also a blindingly efficient homegrown messaging system for the stuff that needed to be really fast. But that was for fairly well-defined use cases. (Latency and concurrency were the key limiting factors.) By default, everything went into the RDBMS-based system. It was inherently more robust, it replaced a whole quagmire of design questions you have to make when using other messaging systems with ACID guarantees, and it made it a lot quicker & easier to do post-mortem analysis of any surprising things that happened in production.