I have quite a good computer with a core i7 processor, GeForce GT750, 16 GB RAM, so Skyrim should run smoothly all the time. However, for the first 5 minutes of gameplay the sound lags, the frame rate drops sometimes and after 5 minutes it runs smoothly. The only reason I can think of is that the gpu takes a while to boot up, and it is just using integrated graphics for the first five minutes but I'm probably wrong. What else can be the source of my problem?
If you are running an ENB, and in some cases a Reshade, they can take a while to load into system memory. Since the GT750 is a laptop model GPU, the VRAM can differ between OEM manufacturers. This can slow down the loading of ENB/Reshade, which can load quite a few texture packages to memory (VRAM).
Alternatively, if you are running a large number of texture mods, this can also cause a slow down in initial load time.
Finally, GeForce Experience is known to cause some crazy ini interference. If you are using it, I recommend uninstalling GeForce Experience, removing your ini files (make sure to take note of any specific mod required ini sequences), and let the game regenerate them. Make sure you then go into your NVidia control panel and reset your 3D Settings for Global and Skyrim back to default. Then work your way back to where you need to be for your graphics setup (e.g. vsync, frame loading, etc.).