Should I buy a NVIDIA video card?
No; you can't really change out the graphics card on an iMac.
But if you could, the answer would be, "it depends, probably not".
Do you plan to use the ray-traced renderer? If so, yes, you'll want an NVIDIA card, but I would recommend against using the ray-traced renderer. It's not a way to speed things up. It was just a way to create 3d geometry out of After Effects layers. The ray-traced renderer is considered a dead and obsolete feature by the After Effects team at Adobe. Especially now that we have Cinema 4D included with After Effects, I don't see a need for the ray-traced renderer. (A great instructional series for C4D Lite is here, if you're interested, [and since that series was released, they've added even more features to the connection between AE and C4D]). Anyway, the ray-traced renderer is just going to slow things down. Use the standard, classic renderer for all your work. I don't even know of anybody who used the ray-traced renderer for a production job even when it was a new feature!
On my Mac, I just ticked the option to never see that warning again and I'd suggest you do the same.
Now, it's unlikely that the ray-traced GPU thing is even an issue for you. Check your composition settings. If they're not set to the ray-traced renderer in the first place (which it won't be by default), that's not your issue. I suspect your issue is that you are thinking of After Effects as a video editing program. After Effects is not a video editor and it doesn't act like a video editor. You have to cache a preview before you can play it back in real time. That's because AE works with video information in a completely different way from video editing software. (I can get into the specifics if you would like, but it's probably fairly boring to you.)
If you're new to AE, I highly recommend going through some basic training materials first before jumping into it. After Effects is very powerful and, thus, very complicated. You can't just poke around in it and learn it like you can with Photoshop or Premiere. Adobe has some good getting started resources on their website. Or you could find a reputable source elsewhere if you would like, but don't just grab random YouTube videos - they are bad in general. I'm particularly fond of Andrew Devis' series of tutorials part of which are linked here. They may be old, but the basic principles of AE are relatively unchanged.
Thank u so much!
I actually using the AE basically for animations, not as a video editor. But thank u for the tips. I will try those things and then keep you guys informed.
You're welcome. happy to be of service.
I just bought a Microsoft Surface Pro 4 i7 256gb 8gb. I installed after effect and when I opened it, it said ray-tracing on a gpu requires an approved nvidia graphics card etc. What does this mean?
It means that the ray-traced renderer won't be accelerated by a GPU.
The ray-traced renderer is considered obsolete. So, if I were you, I would tick the option to never show that warning again and carry on.
OK thank you