1. Futures are not available for every asset.
2. Futures are not listed on every trading platform.
3. Where both futures and perpetual swaps exist, the perp order books are usually deeper and trading volumes higher, which results in faster execution of limit orders.
This deserves merit but I do not have any merit left. But about fast execution time, you have no problem on the exchange future is existing, the volume of coins there are traded in a way that fast execution are guaranteed. That is the reason you will noticed on most exchanges, almost all pairs are not there but just the common ones like BTCUSDT and ETHUSDT. On some exchanges, I see SOLUSDT.