What is the smallest switching time of a transistor ? !
What is the clock speed of the current fastest CPUs?
Clock speed is not a direct indicator of switching speed. The switching speed of a transistor is much much faster than clock speed.
It depends on the type of transistor, the specifications that you need to satisfy, the package (switching on chip or with the package) and whether you are asking about off the shelf or what is only available in the laboratory.
In 2005, I know that the world's fastest transistor was just over 600 GHz. They should have surpassed 1 THz by now as the technology that allowed them to break 600 GHz was expected to bring the speeds into the 1 THz range.
Sure clock speed isn't quite a direct indicator, but it's close enough for physics is it not? The actual switching speed may be faster but on the other hand CPU clocking will have rates in excess of discrete transistors because they are smaller and closely wired to neighboring parts.
On the other hand I'm not sure if the OP was asking what the current fasting switching rates are or if he wanted to know the theoretical minimum switching times. I'm not sure how to answer the last question.