工夫 (gōngfu, here neutral tone in 'fu') means 'time/effort spent.' Don't confuse with
功夫 (gōngfu, 'kungfu / skill') — they're often interchangeable in modern usage but in the phrase
费工夫, the time-and-effort sense dominates. The opposite is
省工夫 (shěng gōngfu, 'to save effort').