The execution unit execute the instruction while the bus interface unit do the fetching and shows the results as an output

In the 8086/8088, the execution unit is the part of the processor known as the CPU. It executes the instructions and generates the results. The bus interface unit, on the other hand, is the part of the processor that handles reading and writing to memory. The two are somewhat loosely coupled, with the bus interface unit attempting to always be ahead of the execution unit, i.e. using the cache, so that the execution unit's wait time is minimized and performance is maximized.

about instruction execution time

It depends on which pentium. The first pentiums had one execution unit. Later pentiums, such as the IV-HT, had two.

Read & Execute instructions provided during execution of Programs.