Optimistic rollups publish transaction results without proof, betting that someone watching will challenge an invalid batch during a dispute window. That window is typically seven days.
The design is cheap and compatible with existing tooling, but the challenge window is the user's problem. Withdrawing to the base layer means waiting it out, or selling your claim to a fast-exit provider.
Example: withdrawing $50,000 through the native bridge takes about seven days at no extra cost. A fast-exit service might front it in minutes for 0.1-0.3%, which is $50-$150. That fee is the market price of the delay, and it widens when the market is volatile.