I just reinstalled FSX steam because some addons arent available in P3d V4...and it runs worse...I'm getting 18 FPS in FSX where I get 60 in P3D in the same situations...I have a new modern computer and I was expecting it to shred FSX...I'll post my specs if needed.

Any ideas?

P3d has received a lot of updates over the years, improving performance pretty significantly versus fsx.  P3d v5 performs much better than V4 for example and V4 is much more refined performance wise vs fsx. 

Also P3DV4 is 64-bit, so optimized for newer processors.  It may may also make better use of the GPS, but don't know for sure.

As for the 18 FPS it depends on setting, scenery, aircraft, etc.  I would think that regardless of CPU the PMDG 737 would be almost unflyable with ORBX's Southern California add-on, especially if using REX textures and AS16.

It is quite true that Lockheed-Martin has made significant improvements in the coding of FSX in order to turn it into a 64-bit flight simulator.  Whereas FSX used the CPU for flight computations and much of the visual rendering, the 64-bit versions of P3D offload more of the visual rendering to the graphics card, allowing the CPU to give priority to the flight computations.

If you're looking for performance with the most FSX compatibility regarding running FSX addons, then P3D v3 would likely be your best bet.

P3D v3 is 32 bit and will by virtue therefore run the majority of FSX 32 bit addons.

There is the issue of getting addons installed in P3D given most FSX apps are looking for FSX named folders, but you can typically work around that by pointing the FSX registry key paths to P3D paths and also drop a copy of the FSX.exe into the root of P3D as there are the occasional addons looking for not only the path, but also confirming the executable exists.

You could also use junction point/s to map the FSX path to the P3D path.

e.g mklink /J "FSX root path\FSX"  "P3D root path\Prepar3D v3"

Personally I did run both FSX-SE and P3Dv3 for a time, however then moved to P3D v4.5 as the majority of planes I owned started arriving with 64 bit counterparts.

Additionally v4.5 being 64 bit has no out of memory issues like that of the 32 bit FSX-SE and P3D v3 and I certainly don't miss the OOMs and the planning and effort to workaround.

The few planes I could not migrate due to no x64 version being available, well... such is life and although a con, having no OOMs is a huge pro.

Cheers

If you're looking for performance with the most FSX compatibility regarding running FSX addons, then P3D v3 would likely be your best bet.

P3D v3 is 32 bit and will by virtue therefore run the majority of FSX 32 bit addons.

There is the issue of getting addons installed in P3D given most FSX apps are looking for FSX named folders, but you can typically work around that by pointing the FSX registry key paths to P3D paths and also drop a copy of the FSX.exe into the root of P3D as there are the occasional addons looking for not only the path, but also confirming the executable exists.

You could also use junction point/s to map the FSX path to the P3D path.

e.g mklink /J "FSX root path\FSX"  "P3D root path\Prepar3D v3"

Personally I did run both FSX-SE and P3Dv3 for a time, however then moved to P3D v4.5 as the majority of planes I owned started arriving with 64 bit counterparts.

Additionally v4.5 being 64 bit has no out of memory issues like that of the 32 bit FSX-SE and P3D v3 and I certainly don't miss the OOMs and the planning and effort to workaround.

The few planes I could not migrate due to no x64 version being available, well... such is life and although a con, having no OOMs is a huge pro.

Cheers