Is it possible to decode and play e.g. 60fps video as 30fps, if computer is too slow for 60fps? I am wondering this, as e.g. YouTube offers some videos 60fps, but computer may be too slow to play them.
It is likely somewhat possible, depending on how the video was encoded. The player can easily skip displaying half the frames but that won't gain us much. What needs to happen is to skip decoding some of the frames.

Modern video codecs nearly all use a mix of I, P and B frames. Most are quite flexible in which frame types are used and only specify what a decoder should be able to handle, leaving the encoder free to pick the most appropriate way to encode a video.

Because of this, even if we want to show only half the frames, we may still end up having to decode more than that. It is possible to build a video stream where every other frame can be skipped because it is not referenced by other frames. But I suspect that most encoders will prioritize encoding efficiency over this rigid requirement. So in practice, we will only be able to skip decoding a smaller number of frames, unless the video was specifically encoded to support this scheme.

The other issue is player support. The player would need to recognize that computing power is insufficient and specifically target half the frame rate. In practice, most that I've seen will skip erratically instead. They expect the lack of computing power to be temporary as that is the most common scenario.

The expected fix for a permanent lack of computing power is to use a lower quality video stream. For devices with well known computing power (eg. phones, tablets, etc), this can be handled automatically by not presenting the high quality alternative. I don't know specifically if YouTube's "auto" quality setting handles this the same way it handles insufficient bandwidth but it could certainly be done and would be a much better solution.

TL;DR: It's possible but usually not worth doing.