1. A computer-implemented method, comprising: rendering, in a first thread in a web browser on a client device, a browser page and content item environments in the browser page according to instructions in a source document, the source document including web page content defining the browser page and the content item environments that are rendered in the browser page, and served from a publisher server; instantiating, from an execution of instructions in the source document in the first thread, a second thread in the web browser, the second thread being separate from the first thread, the instantiation comprising: generating, in the first thread in response to executing the instruction in the source document, a preliminary content item request for second instructions, the second instructions including instructions that when executed by the web browser cause the web browser to initiate the second thread by starting a timer in the web browser and load content item request instructions in the second thread; and executing the second instructions to start the timer; monitoring, in the second thread in the web browser, for an occurrence of a request event, the monitoring comprising monitoring for an expiration of the timer; loading, in the second thread in the web browser, the content item request instructions in the source document in response to the occurrence of the request event, the content item request instructions being instructions to request content items for each of the content item environments based on the expiration of the timer; and generating, in the second thread, a content item request for each of the content item environments to a content item server based on the content item request instructions, the content item server being different from the publisher server; receiving at the client device content items for each of the content item environments in response to the content item requests; and rendering, in the second thread, the content items in the content item environments of the browser page, the rendering of the content items in the second thread being independent of the rendering of the browser page in the first thread; wherein the one or more content item requests comprises one or more advertisement requests.