I think we are slightly going off-topic here but lets say that the faster Sanji gets, the faster the opponent will be that he gets so the speed gets neutralized. We cant expect an opponent thats going to be trashed because of speed. If he takes on Queen, he doesnt need a speed buff, Queen is most likely not a speedster...
During the fight, thye sustain injuries and get exhausted so the speed is to suffer the first most likely and I doubt that we will ever see day long first from protagonists, that just hype for off paneled fights, imo.
At this stage, everything ends up being tied with Haki so I would bet on Oda tying his fire element to Haki somehow or to his Germa modifications/drug that his mother took...
That's a real fight logic tho, they get exhausted and speed would suffer, Oda can have it so or not have it so, it's upto his choice. For eg Luffy vs Katakuri continued for hours and their speed didn't take a hit. Luffy used Snakeman towards the end of fight and Katakuri was keeping up with it. Oda might show it with em barely standing n shit, bt they move fast enough when they need to.
And i didn't say for Speed to be major factor in his victories, speed is just my personal preference to see Sanji get better at.
Tho yes i would agree, faster he gets, faster the opponent will be. Same is true for strength tho, isn't it? The stronger you get the stronger opponent you face, bt it doesn't neutralise, does it?
The diff becomes clear when he doesn't face not so fast opponent, on how much he has progressed, for ex how he showed against Oven or how Nijii moved past the hundred of BMPs.
Another example, Kizaru speciality is Speed and his famous quote of Speed is weight, it make up for his attacks, mobility and can easily dodge attacks like he did against WB. Almost everything bout Kizaru so far has been his speed.