@teachingbirds Hi Isabella,
I just figured out why this was happening for me. The MQTT sensor tracked how much electricity my house was using, and it reported every 15 seconds or so. On my Lovelace home page I had a graph tracking the values using the custom mini-graph card.
Last weekend, I changed the graph to a bar-graph and this was what caused my problem. Removing the graph has restored my home assistant to about 10% CPU.
I hope this helps.