Why is mutation rate high in introns?
Not less efficient, but introns are under less selective pressure than exons. Exons actually encode the protein. A single bp insertion will ruin a mRNA encoding a protein by causing a frame shift in how the sequence is read.

However the same is not true for introns. 1bp insertion even if the intron has function such as transcription or translation factor regulation has a higher chance of not damaging anything important. Simply because such factors are spaced apart and the space is not really important. 

http://www.nature.com/scitable/topicpage/rna-splicing-introns-exons-and-spliceosome-12375
https://www.ncbi.nlm.nih.gov/pubmed/17570818