The Samsung Galaxy S8 is capable of recording slow motion videos in 720p at 240fps frame rate. This is really not great news. I think that many people, including myself, hoped for a much faster frame rate considering the powerful processor utilized in this phone. Just for comparison, Sony's latest flagship smartphone the Sony Xperia XZ Premium is capable of 720p 960fps slow-mo video recording.
I've heard the rumors talking about that the Galaxy S8 will have a 720p 1000fps shooting capability, but this rumor blew out in our face like a big balloon full of water and exploding in not such exciting frame rates :)
I asked myself, if Sony did it with its Xperia XZ Premium Motion Eye camera, why can't Samsung achieve the same thing. The answer for that is because the Sony Xperia XZ Premium utilizes a brand new memory stacked sensor, with a memory module embedded within the sensor itself, and this what allows it to perform so fast. The Samsung Galaxy S8 doesn't feature a memory stacked Exmor RS sensor like the Xperia XZ Premium. This is a Sony technology and one of the most advanced ones you can found right now.
720p240 is the same speed of the S7, nothing changed here. By the way, don't be fooled by many of the fakes Galaxy S8 slow-mo videos that people are posting online. Here is an authentic one shot by Androidworld.it (jump to minute 1:45 to see the slow-mo part).
Now if you wonder what 960fps looks like, so you can understand why so many people are excited about this feature, check this video out.
This is just a huge difference. The 960fps really gives you the option to observe the slightest motion of very fast moving objects or subjects. It maybe a gimmicky feature, but if you love photography, it's a great feature to experiment and get creative with. The 720p240 of the Samsung Galaxy S8 is nice, but doesn't come close to what the Xperia XZ 960fps slow-mo mode offers.
So although it has a very fast Snapdragon 835 chipest, which is the fastest on the market, it lacks the camera'as sensor technology to make this magic happen.