Sometimes Forge gets into a state where it's constantly spiking as shown, and the only way out is to close and reopen it. Normal idle CPU usage is very low, in the single digits percent wise, but when it's spiking like this it shoots up to ~25% and back down. Inside Forge you'll see it briefly freezing during operations. Still playable but preferably not.
I'm just doing Duels and I can't say for certain what causes this, certain cards, certain checks? I've played with and against all kinds of decks and saw it on different patches (I'm currently on 05-May-2023 10:51 , desktop). The spiking continues into non duel screens, like the spell shop, until you close the app.