One herb. One vial. One small step other players will pay to skip. Potion makers need unfinished potions before they add the final ingredient. You can sell them that first step. The trick is not mixing faster. It is buying the ingredients cheaply enough that the finished sale leaves gold after every cost.
OSRS-GE is a free Old School RuneScape Grand Exchange tracker. Prices come from the OSRS Wiki Real-time Prices API and refresh every minute. Every margin shown subtracts the 2% GE tax, capped at 5m gp per item, and respects each item's 4-hour buy limit, so you see the profit you can actually make. Use the calculators to compare flipping, overnight offers, High Alchemy, herb cleaning, decanting, Plank Make, tanning and other money making methods, and read the guides to learn margin checking, how to avoid fake margins and when to trade.