While ForEach-Object is like an order of magnitude slower, we're talking like 100 ms vs 10 ms to iterate through 10,000 items. (https://blogs.technet.microsoft.com/heyscriptingguy/2014/07/...) In other words, you have to have a good number of objects to make foreach meaningfully faster that when you do, you often find that either a) lose time to powershell.exe allocating more system memory to store the collection, or b) the loop itself is two or three orders of magnitude slower already so it's ultimately a trivial optimization.
Also, the foreach statement waits to allocate the entire set to memory ($y in your example) while ForEach-Object begins processing as soon as the first object comes through the pipeline. If you're using the output of a command for $y that returns output with some lag, you might find that ForEach-Object actually turns out to be faster because the first objects start being processed immediately. If you're returning data over a network or dealing with particularly large objects, ForEach-Object can be better.
Finally, the foreach statement itself doesn't work with pipelines either as input or output, so it's not appropriate for a lot of scenarios.