A pool rebalances automatically and always in the wrong direction for a trending market. As one token rises, arbitrageurs buy it out of the pool and leave the other behind, so you end up holding more of the laggard. Compared with holding both tokens in your wallet, you are behind.
The size is knowable in advance for a constant-product pool. A 1.25x change in the price ratio costs about 0.6%, 1.5x about 2.0%, 2x about 5.7%, 4x about 20%, and 5x about 25.5%. It is called impermanent because it reverses if the ratio returns to where you started, and permanent in every other case.
The whole question is whether fees exceed it. A pair that trades heavily but barely diverges, such as two dollar stablecoins, can be comfortably profitable; a volatile pair in a strong trend usually is not. concentrated-liquidity earns more fees per dollar and magnifies this loss in the same proportion.
Related: liquidity-pool, lp-token, concentrated-liquidity, real-yield