How do I design the database to calculate the account balance?

1) Currently I calculate the account balance from the transaction table In my transaction table I have "description" and "amount" etc..

I would then add up all "amount" values and that would work out the user's account balance.

I showed this to my friend and he said that is not a good solution, when my database grows its going to slow down???? He said I should create separate table to store the calculated account balance. If did this, I will have to maintain two tables, and its risky, the account balance table could go out of sync.

Any suggestion?

EDIT: OPTION 2: should I add an extra column to my transaction tables "Balance". now I do not need to go through many rows of data to perform my calculation.

Example John buys $100 credit, he debt $60, he then adds $200 credit.

Amount $100, Balance $100.

Amount -$60, Balance $40.

Amount $200, Balance $240.