You should head home early — it's hot. Mackerel spoils so fast they say it's 'rotting while still alive.'
Literal
Early went-home direction [subject-が] good, hot [because-から]. Mackerel [topic-は], alive-rotting [quotative-とも] said-being extent [particle-くらい] legs [subject-が] fast.
Two related sentences linked by the theme of spoilage. 方がいい ('had better') gives the advice. The second sentence explains: サバ (mackerel) is famous for spoiling quickly. 生き腐れ (いきぐされ, 'rotting while still alive') is a vivid compound. 足が早い literally means 'feet are fast' but idiomatically means 'spoils quickly' — a food-specific usage where 足 (legs/feet) metaphorically represents how quickly something goes bad. ~とも言われるくらい ('to the extent that it's even said...') adds the proverbial framing.