I had the 1.4GHz i5 model briefly.
To give you an idea of the sustained CPU performance, I want to tell you about how well it encoded video. I used OBS Studio to stream video at 720p and 30fps. This is very CPU intensive. The 1.4GHz model was able to maintain the video stream for about 32 minutes with CPU core temperatures steady in the high 70s and low 80s (degrees Celsius).
After 32 minutes, the heat started to overwhelm the cooling capabilities of the chassis, even with the fan at maximum speed, and CPU temperatures started to rise. I stopped when CPU temperatures reached 87 degrees Celsius. I have no doubt that the CPU cores would sustain performance until they reached 100 degrees. Then, they would probably reduce their clock speed to reduce heat output.
If you are interested in longer, sustained CPU performance, I do not recommend the 1.4GHz model.