"How can loops be prevented in a network?"
L2 or L3 loops?
As already mentioned by others, usually a STP variant, if correctly configured and correctly working, will preclude L2 loops. Regarding network performance optimization, much depends on the variant of STP being used. Cisco's per VLAN STP can be used to have different loop free topologies on the same physical topology. That's also true for MST.
Some STP variants "converge" faster than others, which is something else to consider for network performance. I.e. how long a L2 topology change will disrupt traffic.
BTW, your later configuration postings using port-fast, can (rarely) allow a L2 loop to form. Cisco, though, offers other configuration options to reduce this risk even further, as described in other replies.
I believe there are other L2 loop prevention protocols, REP (?), etc. (?), but I've no personal experience using any of them.
At L3, there's no way to preclude a loop, beyond what dynamic routing protocols normally do, although as also mentioned in the other posts, there are "things" in place to mitigate the impact of L3 loops, such as dropping L3 packets that have transited too many routers.
For L3 performance, well there are different methods to trying to optimize performance. The most sophisticated, IMO, is something like Cisco's PfR which can monitor RTT for all flows and attempts to find the best performing path for each flow.
Mention, in other replies, as been made of using storm control. This technology is sort of a "last resort", as it isn't very selective about the traffic is drops. I consider it like Civil War medical treatment, i.e. amputation of a limb that has sustained a gun shot wound.
