The delegation mechanism allows you to have full control over your funds. You delegate only your right to participate in the protocol producing blocks. Thus, when you delegate to a stake pool, you only allow them to produce blocks on your behalf, but you NEVER give up control of your funds.
When you delegate, your funds are NEVER locked, you retain full spending power over your funds at any time. 

The average Return On Investment for staking ADA is currently between 4.5% to 5% per year.

This average will be the same across most stake pools provided that they are minting blocks regularly and have reasonable fees.

Rewards are normally paid out every 5 days at the end of an epoch. However, if this is your first time delegating to any Cardano stake pool, then there is a 15 to 20-day delay from when you start delegating to when you start receiving rewards (and then it is every 5 days from then).

For delegators joining us from other stake pools, there is no gap in rewards as you should continue to receive rewards from your old pool until you start receiving rewards from our pool.