What is the fastest temperature sensor?

A Sous-Vide is just a water bath. Easy to control.

No way you need the "fastest" sensor. Whatever you're using as a control algorithm is probably your problem. Presumably this probe is inside a metal tube and has limited heat transfer.

The main thing from a controls pov is that your sensor should be much faster than the actuator- I don't think you can heat the water from 0°C to 100°C in a minute. If so maybe you should cut back on your heater power from however many kW it is.

1 minute to reach 95% is incredibly slow. Something must be wrong with your sensor. It's just not possible for a TO-92 alone to have such a slow response.

In this Texas Instruments Datasheet for an LM34 (or LM35) in a TO92 package, they give a graph of thermal response in a stirred oil bath, which should be quite similar to your situation:

So in a liquid it will reach 95% in less than ten seconds.

Perhaps you're using a waterproof DS18B20, and it's not thermally connected to the walls of the stainless steel tube. The shiny tube, and thin layer of air, will form quite a good insulator.

They say

• Stainless steel tube 6mm diameter by 30mm long
• Cable is 36" long / 91cm, 4mm diameter
• Contains DS18B20 temperature sensor

So the problem isn't the sensor itself, it's the packaging, which is something you have to solve no matter what sensor you use. The DS18B20 is a great little sensor, mainly because it is reasonably accurately calibrated at the factory. Any more analogue sensor - thermocouples and thermistors, will need special care to calibrate to better than a couple of degrees C.

Try this again with a bare sensor, in a heatshrink tube or a blob of epoxy, or take it apart and fill the tube with oil or glue to improve the thermal conduction to the device.