Front-End Bound metric represents a slots fraction where the processor's Front-End undersupplies its Back-End. Front-End denotes the first part of the processor core responsible for fetching operations that are executed later on by the Back-End part. Within the Front-End, a branch predictor predicts the next address to fetch, cache-lines are fetched from the memory subsystem, parsed into instructions, and lastly decoded into micro-ops (uOps). Front-End Bound metric denotes unutilized issue-slots when there is no Back-End stall (bubbles where Front-End delivered no uOps while Back-End could have accepted them). For example, stalls due to instruction-cache misses would be categorized as Front-End Bound.
A significant portion of Pipeline Slots is remaining empty due to issues in the Front-End.
Make sure the code working size is not too large, the code layout does not require too many memory accesses per cycle to get enough instructions for filling four pipeline slots, or check for microcode assists.