What if, a malicious attacker generate an unlimited number of fake transactions and broadcast it to the bitcoin network. How will the network handle it? In fact, it seems that verifying each fake transaction will take some time and energy.
A normal node which receives an invalid transaction will not relay it, so this attack will only affect the specific nodes that the attacker connects to.
When a node receives a transaction and finds it to be invalid, the node considers the peer which sent the transaction to be "misbehaving" and increases its ban score. When this exceeds a certain threshold, the node will disconnect the peer and refuse any further connections from that IP address for a period of time. Thus, the attacker will only be able to waste a small amount of time for any given node.