From my personal experience, the answer is yes and no, depending on how you look at it.
The Grilley method of Yin, which is the main method taught by many teachers, would be slow in terms of practice speed. This is because the practice style is characterized by poses held for longer periods of time so there's less movement and speed compared to a Vinyasa class.
Hence, Yin isn't quite the class to go to if you're looking to sweat it out. But that isn't to say I've never sweated while doing a Yin practice. I came down with stomach issues during a retreat in 2018 which didn't quite go away after taking medicine. While doing a stomach/spleen practice, sweat emerged curiously from my palms and other parts of the body. I recovered after that.
That brings me to the second point -- effects of Yin practice.
I have definitely seen changes that happened over time such as changes in my toes and nails. For example, I used to have curled toes with dull colored nails. Now they are straighter with a healthier pink shade. I attribute this to frequent practice of tucked toe kneeling. In this sense, one may say the effects are slow because it happened over a long time?
But I have also experienced effects that were quite immediate. The stomach issue example above is one. Students have also told me they experience better sleep the same night after Yin or release of gas from the body (as opposed to bloating) immediately after. I've also heard of a student who has delayed periods found herself having to go to the washroom halfway through class because her period suddenly arrived.
Whenever my body got to a place that it just felt like everything was not right, somehow Yin just gives me a reset that's quite apparent the next day. In this respect, I guess one could say that effects could be fast as well?
Yin looks simple but simple does not mean easy. Neither does it mean it is not a powerful practice.
It does require patience - to stay in the pose, in the room and on the course. Some people love it at first try but for a lot of people, it takes time. If you didn't love your first Yin class, you are not the only one. Then I might tell you: yes it is slow Yoga. The practice is slow. It might take a while to understand and love but please give it some time if you can.
My Yoga journey started on the other end of the spectrum of Yin. I certainly did not expect to come to enjoy this practice or love teaching it but hey, look what happened! You just never know.
If you would like to join me for Yin, please check out the schedule here.