angular2异步http请求

May be I don't understand how async requests should work, but from my point of view 1 ajax or 10 should took almost the same time to be finished. Right now I'm facing a problem, I'm sending 3 ajax requests at the same time, and it took about 3 seconds for the first ajax, 15 seconds for the second, and 18 for the last. But if I send only first ajax - it's took 3 seconds, only second - 15 seconds, only third - 1 second. So, in other words it doesn't matter if I send 3 ajax together

this.contractService.saveContractsForOpportunity(selectedContracts).subscribe(data => {

});
this.contractService.saveContractsForOpportunity(selectedContracts).subscribe(data => {

});
this.contractService.saveContractsForOpportunity(selectedContracts).subscribe(data => {

});

Or if I send it one by one like this

this.contractService.saveContractsForOpportunity(selectedContracts).subscribe(data => {
    this.contractService.saveContractsForOpportunity(selectedContracts).subscribe(data => {
       this.contractService.saveContractsForOpportunity(selectedContracts).subscribe(data => {

       }); 
    });        
});

It tooks the same time to be finished

May be it's only about backend or server configuration?

Most likely your problem lies on the server side. Does the server allow handling multiple requests from the same IP in parallel?