Steric-blocking antisense oligonucleotides (ASOs) have been developed for the treatment of Myotonic Dystrophy type 1 (DM1), a gain-of-function disease caused by the sequestration of MBNL splicing factors on an expanded repetitive region within the DMPK gene. CAG-repeat ASOs are highly specific to intracellular DMPK mRNA targets and are effective at rescuing DM1 spliceopathies. In humans, systemic injections of naked oligonucleotides did not reach clinically relevant intracellular drug concentrations; thus, conjugation strategies ranging from antibodies to small peptides are now employed to enhance oligonucleotide delivery to cells. The cyclic cell-penetrating peptide conjugation method used here is a fast-acting delivery system that rivals the cellular delivery speed of small molecules, while maintaining its molecular target-specificity. In this work, we show that a single systemic administration of the endosomal escape vehicle (EEV)-conjugated CAG repeat blocking phosphorodiamidate morpholino oligomer (PMO) fully rescued disease phenotypes in HSALR mice after 1 week and partially rescued phenotypes as early as 1 day. Treatment with this fast-acting compound, in combination with metabolic labeling, revealed that discrepancies in rescue levels and apparent delay in total RNA rescue can be explained by transcript half-life and MBNL responsiveness of the splice events. As more oligonucleotide therapies enter clinical trials, understanding how PMOs influence gene expression in DM1 provides insight into possible early markers of improvement in humans, where the disease is more variable and outcomes can be difficult to measure.