Foreach-Object and Where-Object are among the most frequently used pipeline cmdlets, yet they tend to be very slow with a large number of iterations.

A classic foreach loop in comparison to the pipeline often is 100x faster.

Suspected Design Flaw? Reason for Slowness:

That's because both take a scriptblock and invoke it via InvokeReturnAsIs() per each iteration. This prevents code and compiler optimizations.

Technical Improvement - how to make Foreach-Object as fast as foreach loops:

By replacing scriptblock invocation via InvokeReturnAsIs() by a steppable pipeline, Foreach-Object and Where-Object can be just as fast as foreach loops.

Prototypes and Explanation

A detailed explanation, use-cases, and prototypes are available here:
https://powershell.one/tricks/performance/pipeline