Newer codecs has generally introduced more ways a video can be encoded, so that the encoder need to work much harder to encode a video, so that it actually achieve the gains that the newer codec allows much more processing will be required. Decoding on the other hand, will mostly stay the same or increase only slightly. It's not likely do decrease though, so if you struggle playing av1 today, you will also struggle with av2.

For encoding, you can always write a simple encoder that use only the features that were present in mpeg2, and it will be about as efficient as mpeg2 as well. Newer codecs has more features that allows more efficient encoding, at the cost of more processing.