How do you lower frames to 48 fps from 144 fps in SVP4?

Just to make sure, your CPU is nearly maxed out when trying to interpolate to 144fps, correct?  If it is not maxed out, then you should first probably try to manually increase the amount of threads that SVP uses (Application settings -> Additional options -> Processing threads) since it tends to be overly-conservative and  therefore leave performance on the table; fyi set it to the smallest value that gives you perfectly smooth performance (don't just max it out because, if you have perfectly smooth performance with 15 threads, then setting 23 threads will give higher CPU utilization without any benefit)

But even if your CPU utilization is not maxed out, sometimes you can still have wonky performance issues that can only be solved by doing one or more of the following:

1. If you use MadVR, enable "use direct3d 11 for presentation"

2. If you don't use MadVR but do use MPC-HC, enable "D3D Fullscreen" in 'Options' under "Output"

3. In LAVfilters (particularly the one built into MPC-HC if you use that), set your decoder to "DXVA2 (copy-back)"

4. If all else fails, try to disable GPU acceleration in SVP itself (Application settings -> GPU acceleration)

If none of the 4 suggestions listed above fixed your issue, then at least make sure you re-enable GPU acceleration in SVP; other than that the other settings can be left alone or changed according to your preference.

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Anyway, to actually answer your question, the only way to do that in SVP 4 Free is to actually set your monitor to 48Hz. In order to do this you may need to make a custom resolution.

It is important to note however that, in SVP, doing 5x and 6x interpolation is considerably more intensive than 3x and 4x.  While 2x is indeed the least intensive, even mid-level systems should have no problems with 3x or 4x, so you may want to try those out before dropping all the way down to 2x.

If you use MPC-HC, I would also highly recommend using its built-in automatic resolution/refresh rate changer; info on setting it up (along with configuring SVP to activate even on high frame rate videos) can be found in this thread:
http://www.svp-team.com/forum/viewtopic.php?pid=56025

If you do not use MPC-HC, I believe MadVR has a similar automatic resolution changer, but I have no experience with it.

DXVA2 HW decoding is proposed to offload the CPU for SVP, I think.

Which should I use then?

I have my Intel iGPU disabled, but I have an Nvidia GPU.

CPU usage is usually around 35-40% but GPU is "maxed" at 83-87% because I'm using madVR. Anything more GPU than 83-87% and I get frame drops so I could benefit by offloading something to the CPU or using a more efficient decoder for the GPU.

I actually don't know how to switch between them. I'm using Shark007 codecs, if I select anything else from what's already selected SVP doesn't activate.

If I select LAV and change the decoder it it says it's active but SVP doesn't detect it.

I tried doing this, but it uses FFDSHOW Raw instead. If I move that down or block it, SVP doesn't detect the video. How can SVP can work without any FFDshow stuff? So far I haven't been able to make that happen.

But I really don't know how this works. That's the downside of Shark007 codecs. It works so well with everything you don't have to fix anything and learn almost nothing.
So I don't know the "chain of operation" of all these settings or even what these things are actually for...

Yea I figured that out and it says they're active.
HOWEVER, if I use the LAV video decoder instead of the ffdshow video decoder SVP will not detect the video.