mempool congestion, your transaction is basically waiting in line like squeezing into a subway: first being stuffed into the pool and left to sit, miners/validators glance at the fee, if it's too low they pretend not to see. Then various routers and packers start "picking vegetables," conveniently snatching arbitrage opportunities (don't ask, just MEV on the spot), if you happen to be stuck in the middle, you might wait a long time, get dropped, or if someone else did the same operation first, when you try to re-enter, all that's left are slippage and failure fees.

Recently, there have been a lot of guesses about testnet points and whether the mainnet will issue tokens... congestion becomes even more obvious: everyone rushes together, on-chain is like a cafeteria opening for meals, only the fast and those willing to pay more are recognized at the window. To put it simply, don't believe in the narrative that "once I submit, it will definitely be executed," it's only probabilistic: pay enough fee + avoid crowding at the same second as the majority, then your success rate is higher; otherwise, just consider it paying tuition to the chain, for now.