In a system in which documents are generated dynamically in response to user requests, historical data is collected regarding data retrieval subtasks, such as service requests, that are performed to generate such documents. This data is used to predict the specific subtasks that will be performed to respond to specific document requests, such that these subtasks may be initiated preemptively at or near the outset of the associated document generation task. A subtask that would ordinarily be postponed pending the outcome of a prior subtask can thereby be performed in parallel with the prior subtask, reducing document generation times. In one embodiment, the historical data is included within, or is used to generate, a mapping table that maps document generation tasks (which may correspond to specific URLs) to the data retrieval subtasks that are frequently performed within such tasks.