Terrible police responses to mass shootings/active shooters

The only exceptions is shootings that have happened before active shooter tactics were put into the place, like Columbine for example.

Stoneman Douglas High School shooting - None of the BSO deputies that were responding to the shooting at Building 12 have rushed into the building. Not even the school resource officer, Scot Peterson, who just stood right outside of the 1200 building while the shooting was happening. It took officers 11 minutes for them to enter the building, which by then Cruz had already left the building 5 minutes ago.
If you want a more in-depth look at the police response to the Parkland shooting, I recommend reading this page made by Sun Sentinel: [You must be registered and logged in to see this link.]

2012 Empire State Building shooting - 9 bystanders were wounded by the NYPD after fatally shooting a gunman who earlier shot and killed a former co-worker outside of the Empire State Building.

Manny's Blue Room Bar shooting - Security guards shot at a gunman who opened fire at a bar and injured 3 people. One of the security guards, Jemel Roberson, apprehended the suspect outside of the bar. Once the Midlothian Police arrived, one of the officers shot and killed Jemel, believing he was the gunman.

Christchurch mosque shootings - It took officers 10 minutes for them to arrive to the Al Noor Mosque after 42 people were killed, and it took them even longer for them to apprehend Brenton Tarrant even after the attack at the Linwood Islamic Centre. They did however prevent a third attack at a third mosque.

Saying about terrible police responses, Zhao Xiaodong comes to mind.

He had already killed some people before the bus siege, but the bus siege is the main part of the disastrous response from police:

At 1 p.m. Zhao boarded a bus at Podi (坡底村), a village one kilometre north of the bridge, and a quarter hour later it approached the structure from the northern end, where it was stopped by police for inspection. The county cadre soon spotted Zhao and in panic rushed out of the vehicle, yelling that he was on the bus, whereupon Zhao drew his pistol, shouting: "Quick, drive! Drive!", at the same time firing a shot at a police officer, injuring him. The bus driver tried to use the gunman's distraction to escape, however, Zhao noticed this, shot him as well, and afterwards threatened the other passengers that he would kill anyone who dared to move. He then put himself behind the steering wheel and drove towards the bridge, while one of the guards outside opened fire with a Type-56 assault rifle, hitting him in the cheek and penetrating several of the bus's tires. 600 metres down the road Zhao finally lost control over the vehicle and crashed into the bridge railing, leaving it stuck, with the right front wheel hanging off the bridge.

Soon the bus was surrounded by members of the police and armed police, with a squadron approaching it, sheltered behind a postal car. Halting 30 metres from the bus they ordered Zhao to drop his gun, however, he opened fire and threw a bomb at them, causing hand injuries to one of the officers. In the ensuing gunfight with police, Zhao was seriously injured, but still continued to shoot at his besiegers, while several of the passengers tried to seize the opportunity to escape. Zhao noticed this, shot several rounds at them, forcing them to take cover behind their seats, and then resumed firing at police, injuring the squadron leader in the process. Witnessing the wounding of their leader and hearing Zhao shooting the passengers, the armed police forces indiscriminately opened fire at the bus and continued to shoot, until they were hastily ordered to stop by their team leader and instructor. Shortly after the shooting stopped a male passenger jumped out of one of the bus windows.

At approximately 1:40 p.m. the director of the Jili branch office, the city police captain, and others arrived at the scene, ordered to cease all shooting, and to get a fire truck and a truck loaded with cement bags. At 2:20 p.m. members of the armed police eventually used the cement truck as cover to approach the bus, while the fire truck sprayed it with foam, to impair Zhao's view. Zhao continued to shoot at both vehicles, while the armed police returned fire. During the gunfight a second passenger managed to escape. Ten minutes later, when the police officers saw no more movement inside the bus, they stormed the vehicle through the rear window and discovered Zhao's body, who had been hit by sixteen bullets. By the time of his death he had only three rounds of ammunition left. The surviving passengers were rescued and at 2:40 p.m. the siege was finally declared over.

Overall 68 members of the police and armed police participated in the siege, during which one police officer and two members of the armed police were wounded. In total they fired 528 rounds at the bus, leaving all of its windows broken and 375 bullet holes in the vehicle's body. Of the 17 people who were originally in the bus beside Zhao, Zhao killed five and wounded three, while seven others were killed by bullets fired by police and armed police, and the two remaining escaped by jumping out of the vehicle. Additionally, ten other people were accidentially injured during various stages of the siege.

Source: Amok Wiki's article on Zhao Xiaodong

Las Vegas and Uvalde.

Derrick Bird's rampage through Whitehaven had the police almost chasing him, but he managed to get away and by the time he got to Seascale, he was over 7 minutes ahead.

Uvalde, the delayed response of the police was unbelievable.

Nakhon Ratchasima shooting I think, man just started a war against the whole world for around 16 hours. What police can do against armed militant with machine guns