This is just Rooftop Kaido all over again, when will you guys learn that no arc villain goes all out in their first fights? They sandbag and show a fraction of their full power and then gradually escalate.
A serious Imu who probably still isn't even close to using his full power took care of Loki in moments, there's just no denying what chapter 1187 explicitly shows.
How do you know Luffy performed worse? Have you seen the chapter? All we know is the end result, which is that Luffy apparently gets knocked out, just like Loki was.