It depends a lot upon where the processing is happening. For example, you could do something where all the data is pre-processed and you're just blasting bits into a GPIO register with a pair of move instructions. In which case you could get north of 60MHz, but I think that's sort of cheating - you'll run out of pre-processed data pretty quickly, and then you have to take a delay to generate more data.

The 25MHz number I cite as the performance expectation is "relaxed": I don't want to set unrealistic expectations on the core's performance, because I want everyone to have fun and be happy coding for it - even relatively new programmers.

However, with a combination of overclocking and optimization, higher speeds are definitely on the horizon. Someone on the Baochip Discord thought up a clever trick I hadn't considered that could potentially get toggle rates into the hundreds of MHz's. So, there's likely a lot to be discovered about the core that I don't even know about, once it gets into the hands of more people.