"Computing power" means simply the ability to perform arithmetic and logical operations within a computer. Two common measures of computing power are clock speed (e.g. "2.8 GHz") and floating-point operations, known by the acronym FLOPS (e.g. "the Intel Core i9 Extreme Edition processor can perform 1000 gigaflops/second.") A 2.8 GHz processor has twice the computing power of a 1.4 GHz. processor. A 200-gigaflop processor has twice the computing power of a 100-gigaflop processor. Your quotation sounds as if it comes from the 1960s and 1970s. Personal computers did not exist. Computers were too expensive to give a single person an entire computer. In order to provide an interactive computing experience, computer scientists developed "time-sharing systems," in which a single computer was connected to many users, and switched rapidly back and forth between each of them. 50 users might each use 2% of the machine's computing power.

It means "the use of computers". It is not important for the users if this is one computer, one hundred computers or one hundred thousand computers

This refers to a group of people that are doing a job or are problem solving.

Well, your computer has been designed to solve many equations, mathematical additions, etc etc. This is its "computing power".

"Computing power" - the ability to calculate and solve things by people or machines. In the old days, it means only people. The original definition of a "computer" is a person doing calculations such as an actury. However, the meaning has changed nowadays since machines become more efficient at doing certain calculations... Hence we have modern expressions such as "He's like a computer in calcuation"; meaning that he is so efficient and accurate with numbers.