The phrase "didst a slow burn" implies a gradual build-up of anger and frustration. There are several synonyms that can be used to convey a similar emotion. Some of these synonyms include simmer, seethe, smolder, fume, and boil. Each of these words suggests a level of intensity behind the anger, with simmer and seethe highlighting a slow burn, and smolder, fume, and boil indicating a more intense and explosive reaction. Using these synonyms can help to add depth and nuance to writing, allowing the reader to better understand the emotional state of the character.