Electronic – Clock generator is not oscillating on the given XTAL’s frequency

I think that your question is aimed at the program execution speed, and not the clock frequency.

This link has a simple way to explain pipeline. But the picture below (from the site) brings the base information on how the pipeline structure can hurt the execution speed:

From the picture you can see that the pipeline takes advantage of executing two or more tasks simultaneously. The problem is that the period of execution for one cycle is dependent of the longest task in the pipeline. If lots of long tasks enters in the pipeline, the whole chain may suffer the delay. Among these kind of tasks, memory instructions are one of the longest.

If all the memory access are made scattered through the code, the processing time would be higher. If the accesses are grouped, the fast instructions would be executed without the interference of the slower ones.

Other problem that may arise is the junction of pipeline and branch prediction. The prediction tries to guess which is the more probable result from an if instruction and starts to evaluate the guessed path from code even before the result is known. All these instructions are therefore pipelined through the processor. If the guess appears to be wrong, the processor must wait until the pipeline buffer to clear before starting to evaluate the correct path.

A small micro would do this easily. You can set up a capture compare peripheral to count input pulses and read/clear at a regular interval (e.g. 50ms - use a timer interrupt for this)
Then write result (after multiplying by necessary value - e.g. in above example: capture_result * 20 = Hz) to whatever display you have in mind.

A non-micro solution could be produced with some discrete logic ICs, there are quite a few designs floating around on the web. It would be a pretty "old school" solution but may be fine for a one off home project (and cheap) Here is a 0-40MHz example (from this page)

For higher frequencies a small CPLD/FPGA would work well and would be a quick/easy HDL program to write - may be an idea if you have a small dev board lying around.

There are almost certainly dedicated ICs/Modules available if you have a look around. Most DMMs probably use an ASIC of some sort to provide multiple functions, though some of the cheaper models may use off the shelf stuff.