Invisibility by Speed just means that the speedster is faster than the enemy's perception.
### In East Blue, during that time, Kuro is moving so fast he seems invisible. People at that stage is low tier, so they can't see Kuro.
- Which means Kuro has Mid tier speed, his enemies are just Low tier.
### In Enies Lobby, Luffy upgrades his perception from Mid tier to High tier. Mid tier Luffy couldn't see Blueno, but later he can.
- Which means Blueno has High tier speed, Luffy with Mid tier perception can't see him, but Luffy with High tier perception can.
### In WCI, Oven, a High tier, can't see Sanji.
- Which means WCI Sanji has Top tier speed. This is why he can dodge Katakuri, another Top tier.
### In Onigashima, King and Queen, a Top tier, can block and attack Sanji normally. It is until after Exoskeleton, that a Top tier like Queen can't perceive Sanji.
- Which means Wano Sanji has speed beyond Top tier. So unless it's an Emperor level characters, they can't see Sanji. Sanji's speed will be on par with them. Only Emperor level characters that specialize in perception can beat him, maybe like Kizaru.
In conclusion, just like how Wano Zoro has Emperor level attack, Wano Sanji has Emperor level speed, while Luffy has both.
### In East Blue, during that time, Kuro is moving so fast he seems invisible. People at that stage is low tier, so they can't see Kuro.
- Which means Kuro has Mid tier speed, his enemies are just Low tier.
### In Enies Lobby, Luffy upgrades his perception from Mid tier to High tier. Mid tier Luffy couldn't see Blueno, but later he can.
- Which means Blueno has High tier speed, Luffy with Mid tier perception can't see him, but Luffy with High tier perception can.
### In WCI, Oven, a High tier, can't see Sanji.
- Which means WCI Sanji has Top tier speed. This is why he can dodge Katakuri, another Top tier.
### In Onigashima, King and Queen, a Top tier, can block and attack Sanji normally. It is until after Exoskeleton, that a Top tier like Queen can't perceive Sanji.
- Which means Wano Sanji has speed beyond Top tier. So unless it's an Emperor level characters, they can't see Sanji. Sanji's speed will be on par with them. Only Emperor level characters that specialize in perception can beat him, maybe like Kizaru.
In conclusion, just like how Wano Zoro has Emperor level attack, Wano Sanji has Emperor level speed, while Luffy has both.