First off, why?
Axxtentacle only arguement for Deku "Keeping up with Metal" is the Analytical Prediction
He has yet to use anything else to say Deku can keep up with:
1. Goes 0 to 100 in Speed far faster than Deku because of the scaling to Sonic's Acceleration
2. Creates AfterImages
3. Has a Speed Amp technique he can use on the ground.
4. Can predict Deku's moves without thinking, while Deku needs to think about his opponent to predict them
5. And has better mobility in the air to fly, due to the nature of both of their abilites to fly, Deku needs to constantly be on the move to fly with Metal can choose to hover.
And Metal has feats of flying to a high altitude than Deku and still fly perfectly fine without exhaustion, lasted for an entire day.