I'm pretty sure the camera sensor bottlenecks the phone's ability to record at higher framefrates. The OnePlus 3/T uses the
Sony IMX298 camera sensor, while phones like the Nexus 6P use the more powerful
Sony IMX377 sensor that is actually designed to record at higher framerates at lower resolutions.
The specs sheet for the IMX298 sensor from Sony themselves state that the sensor can only achieve up to 120 fps at 720p. Even lowering the resolution during recording will not overcome the limitations of the sensor. This isn't the type of limitation that can be overcome with software. No other phone that I have seen using the Sony IMX298 sensor is able to record above 120 fps either.
When one designs a flagship phone at half the price of other flagships, some things must be cut. OnePlus simply gave priority to the CPU/RAM over the camera sensor.
---------- Post added at 03:22 PM ---------- Previous post was at 03:02 PM ----------
BTW, the camera app in CR Droid may show a 240fps option, but has anyone confirmed that it actually works? Custom ROMs are often ported across multiple devices and I doubt developers will go through every settings menu in each app to see that has become redundant.