Cygwin's pdflatex: i5-3340M runs 2-3x faster than i5-8250U

The results you are seeing are absolutely correct.

Yes, your newer processor is faster... However, only in multi-threaded applications. In single threaded applications running on a single core, your newer CPU is actually slower. It runs at 1.6 GHz, which is 60% slower than the older CPU's 2.7 GHz.

You could run 4 instances of the application and force them onto 4 different cores then you would come out ahead. However, in a one to one comparison, the older CPU is faster.

I havent looked at Cygwin in ages, but when I last did, the applications were single threaded. I assume they still are. Someone can correct me if this has changed.

To maximize your performance you will want to check the windows power settings. Many laptops come by default with a balance or power-saver plan enabled which will maximize the battery life of the device by limiting the overall performance.

In recent versions of windows, you can click the battery icon in the system tray to access a slider. Move the slider to the Best Performance slot.

If that does not help, or the slider is not present then go to the power options in control panel via Win+X -> Power Options (O) -> Related Settings -> Additional power settings. Look for a plan labeled High Performance and select that plan. If the plan does not exist, you can create it using the Create a power plan link on the left side menu.

You may want to change the power plan back to what it was previously when you're done with your task to preserve battery life. If you need to change plans often, it is possible to create desktop and/or keyboard shortcuts for power plans to allow for fast switching.