A few problems with the SFM

[Aa21]

First off, cp_mountainlab is only loadable when I launch SFM in the Source Filmmaker folder in the C:/D: drive,but not when I launch it from the Steam app. When I launch SFM from the steam app, it crashes when loading cp_mountainlab. I think it's because I can right click to use my Nvidia graphics card instead of another lower-quality graphics card to load SFM if I launch it straight from the folder. But what's weird is that I have already set in my graphics control panel to launch SFM with my Nvidia card,so launching SFM from steam app should be the same as clicking it in the folder. But what I have been experiencing these days showed there's a difference between them and I don't know why.
EDITTED: nothing wrong with the graphics card, I tried to right click in the SFM folder to use the lower-quality GPU and the map loads just fine, though the framerate is ♥♥♥♥ as 6-7. Now the problem is that why can't I load maps properly if I launch SFM through Steam?

Second, while I think I have got the preferred specs for SFM (or have I?), my framerate is still 12-13 while the tutorial video on YouTube has 30-40. Although it's still playable but the smoother the better ,right?
Computer Specs:
Intel i3-2350M
Nvidia GeForce 620M
8GM of ram


Any help is very appreciated.