punishingly       
adverb uk/ˈpʌn.ɪ.ʃɪŋ.li/ us/ˈpʌn.ɪ.ʃɪŋ.li/
 繁重费力的,艰难的
in a way that is very difficult to do or cope with, or makes you feel very tired
  a punishingly heavy workload  繁重的工作量 
 Instead of gaining momentum, the saga turns punishingly dull.  这部传奇不但没有愈加精彩,反而变得乏味无奇。 
 See: punishing
 - More examples
 - The month of January was punishingly cold.
 - Punishingly hard work and financial instability took its toll.
 - Our property taxes are punishingly high.