Based on the level of respect or formality, you can say "Can you speak slowly?" in Korean in 3 ways:

• Informal: cheoncheonhi malhae-julrae? (천천히 말해줄래?)
• Formal: cheoncheonhi malhae-jusigetsseoyo? (천천히 말씀해 주시겠어요?)
• Very formal: cheoncheonhi malsseumhae-jusigetsseumnikka? (천천히 말씀해 주시겠습니까?)