That's not right, all behaviors, including physics, should already be framerate independent. You might have some other issue.
Note that there is a minimum framerate of 30fps.
At very low framerates, dt can become very large. For example, at 5 FPS, dt is 0.2. An object moving at 500 pixels per second is therefore moving 100 pixels per tick. This can cause it to "teleport" through walls or miss collisions with other objects.
Games are usually unplayable at such low framerates, but it is even worse if they become unstable like that. To help the game stay reliable even at very low framerates, Construct 2 does not let dt get larger than 1/30 (about 0.033). In other words, below 30 FPS, dt stays at 0.033. This does also mean below 30 FPS the game starts going in to a slow-motion effect (described earlier as one of the issues of framerate dependent games), however this is usually a better result than the "teleporting objects" problem.
If you want to set a different limit to 1/30, you can use the system action Set minimum framerate.