The only template supplied with PFPX is for the QualityWings 757-200 with PW engines.
You can add additional profiles which can include RR from AirlinerPerformance.net or from the download section here.
These are placed in the aircrafttypes folder and selected by 'Add new aircraft' option and not from template. You may need to adjust weights and capacities to suit your add-on.
Use the PFPX support board for further help.
Your processor is at the very minimum capable to run the CRJ, so it's already struggling, plus you're running on a integrated graphics card and this also adds to the overall equation. FSX will begin loading airport graphics at about 18nm, and it loads more and more airport graphics while continuing to load autogen graphics, all putting an increased load on your processor which is not only handling all the flight sim computations but the vast majority of graphics processing (something it's not designed to do). Then you select the approach mode which entails further processing. So what you're likely seeing is the overall combination of the processing demands. In contrast, running on a i7-4770k with either a GTX960 or GTX 1080 I don't see the problems that you're experiencing.
Yes, PFPX. Open app, clean slate - no plans, no aircraft in database.
1. I select Aircraft data from top left of screen.
2. Click Aircraft database. (it is empty)
3. Select New as under templates, my aircraft is not listed.
4. Under General, I fill in Registration, Tail Number
5. For type, there is only a drop-down so I select Boeing 757-200
6. Engines - all there is there is the PW2037