Renowned filmmaker / DP and all around nice guy Philip Bloom has been having fun with both the Sony RX10 II and RX100 IV this past few days. He has tested the slow motion and 4k modes extensively and has done a fantastic rundown of the usability, performance and pitfalls on using the cameras. There are a lot of interesting tidbits that He discovered such as the 240fps NTSC and 250fps PAL modes are nearly HD in quality and extremely usable very close to what the much more expensive Sony FS700 camera achieves.
When recording the 2 second quality priority mode it records back to the SD card in Real time that means a 2 second video at 240fps will take 20 seconds if saved as 24p for the frame count of 480 frames duration. At 480fps for 2 seconds of 960 frames it will take 40 seconds of write time to the SD card. This is in line with most slow motion cameras and it is a point to be considered while using the HFR modes. You will have to plan ahead as the camera will be unavailable for use for quite a long time before the next shot is ready to be taken. This is a normal thing in most high speed cameras.