SPWeb.ProcessBatchData method to improve SharePoint list operations performance

The common way to add or update multiple items in SharePoint list is SPListItem.Update. You can an existing item or add a new item using SplistItemCollection .Add(). 

Sample Code Snippet:

for (int itemCount = 0; itemCount < 100;s itemCount++)

{

SPListItem newItem = listItemCollection.Add();

newItem.Update();

}

Using Batch Update:
ProcessBatchData method takes XML format as input and it will directly communicate with SharePoint content database.

StringBuilder query = new StringBuilder();

query.Append("<?xml version=\"1.0\" encoding=\"UTF-8\"?><Batch>");

for (int itemCount = 0; itemCount < 100; itemCount++)

{

query.AppendFormat("<Method ID=\"{0}\">" +

"<SetList>{1}</SetList>" +

"<SetVar Name=\"ID\">New</SetVar>" +

"<SetVar Name=\"Cmd\">Save</SetVar>" +

"<SetVar Name=\"{3}Title\">{2}</SetVar>" +

"</Method>", i, listGuid, someValue, "urn:schemas-microsoft-com:office:office#");

}

query.Append("</Batch>");

spWeb.ProcessBatchData(query.ToString());

Conclusion:
If you have to update a larger number of items its highly recommended to not use the Update method on every item. Instead – use the batch update function ProcessBatchData provided by SPWeb.

Reference Links:

http://msdn.microsoft.com/en-us/library/cc404818.aspx