Collect Kokos. LOTS OF THEM. Then go to the Elder Koko to deliver them and upgrade your Speed (or Ring Maximum).
If you have upgraded your Ring Maximum, you can talk to the Hermit Koko to transfer those upgrades over to Speed (and then back again) to max out your Speed faster.
If you haven't already maxed out, you can do this extremely easily in Final Horizon's Another Story (the DLC). When you get to play as Sonic again (after playing through the first part of Amy, Knuckles, and Tails' stories), wait for a Starfall then collect all of them like mad. Instead of Fishing Tokens, you will earn multiples of a Koko, Blue Seed of Defense, or Red Seed of Power). The amounts given are vastly multiplied, so you can potentially max out at least one stat from a single starfall if you collect enough of a given item. It may take more than one starfall, but it is still faster than collecting single items like normal.