Required for normal mitochondrial respiration. Increases steady-state levels and half-lives of a subset of mature mitochondrial mRNAs MT-ND2, MT-ND3, MT-CYTB, MT-CO2, and MT-ATP8/6. Promotes MT-CO1 mRNA translation and increases mitochondrial complex IV assembly and activity. Expression detected in spleen, testis, colon, heart, smooth muscle, kidney, brain, lung, liver, brown and white adipose tissue with highest expression in testis and smooth muscle. RAP domain is required for FASTKD3 function in mRNA stability and translation. Belongs to the FAST kinase family.