We have a medium to semi-large site (10k+ content nodes) and we're having issues with our site going down whenever there are 10-15 editors working concurrently. Below is a portion of the log. The only thing the log really shows is a multiple attempts to login by all the users logged in and it keeps going until the app pool crashes. The mass amount of login attemtpts is usally preceeded by a SQL error.

Setup:
Windows Server 2008 (with more than enough CPU and memory to handle the load), SQL Server 2012, Umbraco 6.1.6, Using Active Directory for user auth. We're not doing any load balanacing. The Umbraco instance and database are on the same server.

2015-02-10 16:13:48,599 [125] INFO  umbraco.BasePages.BasePage - [Thread 37] User John Doe (Id: 501) logged in
2015-02-10 16:14:03,659 [125] ERROR umbraco.DataLayer.SqlHelper`1[[System.Data.SqlClient.SqlParameter, System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]] - [Thread 74] Error executing query SELECT id, createDate, trashed, parentId, nodeObjectType, nodeUser, level, path, sortOrder, uniqueID, text FROM umbracoNode WHERE id = @id
System.Threading.ThreadAbortException: Thread was being aborted.
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior)
   at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(SqlConnection connection, SqlTransaction transaction, CommandType commandType, String commandText, SqlParameter[] commandParameters, SqlConnectionOwnership connectionOwnership)
   at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, CommandType commandType, String commandText, SqlParameter[] commandParameters)
   at umbraco.DataLayer.SqlHelpers.SqlServer.SqlServerHelper.ExecuteReader(String commandText, SqlParameter[] parameters)
   at umbraco.DataLayer.SqlHelper`1.ExecuteReader(String commandText, IParameter[] parameters)
2015-02-10 16:17:17,690 [125] INFO  Umbraco.Core.Publishing.PublishingStrategy - [Thread 81] Content 'Home' with Id '3702' has been published.
2015-02-10 16:18:39,245 [125] INFO  umbraco.BasePages.BasePage - [Thread 28] User John Doe (Id: 501) logged in
2015-02-10 16:18:42,003 [125] INFO  umbraco.BasePages.BasePage - [Thread 76] User John Doe (Id: 501) logged in
2015-02-10 16:18:57,473 [125] INFO  umbraco.BasePages.BasePage - [Thread 76] User John Doe (Id: 501) logged in
2015-02-10 16:20:22,613 [125] INFO  umbraco.BasePages.BasePage - [Thread 34] User John Doe (Id: 75) logged in
2015-02-10 16:20:37,385 [125] INFO  umbraco.BasePages.BasePage - [Thread 37] User John Doe (Id: 75) logged in
2015-02-10 16:20:37,389 [125] INFO  umbraco.BasePages.BasePage - [Thread 28] User John Doe (Id: 75) logged in
2015-02-10 16:20:38,934 [125] INFO  umbraco.BasePages.BasePage - [Thread 61] User John Doe (Id: 75) logged in
2015-02-10 16:20:40,964 [125] INFO  umbraco.BasePages.BasePage - [Thread 66] User John Doe (Id: 75) logged in
2015-02-10 16:20:43,910 [125] INFO  umbraco.BasePages.BasePage - [Thread 18] User John Doe (Id: 619) logged in
2015-02-10 16:20:48,230 [125] INFO  umbraco.BasePages.BasePage - [Thread 6] User John Doe (Id: 75) logged in
2015-02-10 16:21:14,690 [125] INFO  umbraco.BasePages.BasePage - [Thread 73] User John Doe (Id: 627) logged in
2015-02-10 16:21:17,295 [125] INFO  umbraco.BasePages.BasePage - [Thread 75] User John Doe (Id: 627) logged in
2015-02-10 16:21:22,577 [125] INFO  umbraco.BasePages.BasePage - [Thread 88] User John Doe (Id: 648) logged in
2015-02-10 16:21:24,894 [125] INFO  umbraco.BasePages.BasePage - [Thread 63] User John Doe (Id: 648) logged in
2015-02-10 16:21:27,199 [125] INFO  umbraco.BasePages.BasePage - [Thread 67] User John Doe (Id: 627) logged in
2015-02-10 16:21:27,702 [125] INFO  umbraco.BasePages.BasePage - [Thread 24] User John Doe (Id: 648) logged in
2015-02-10 16:21:28,466 [125] INFO  umbraco.BasePages.BasePage - [Thread 74] User John Doe (Id: 592) logged in
2015-02-10 16:21:30,076 [125] INFO  umbraco.BasePages.BasePage - [Thread 18] User John Doe (Id: 627) logged in
2015-02-10 16:21:31,197 [125] INFO  umbraco.BasePages.BasePage - [Thread 100] User John Doe (Id: 627) logged in
2015-02-10 16:21:31,281 [125] INFO  umbraco.BasePages.BasePage - [Thread 30] User John Doe (Id: 592) logged in
2015-02-10 16:21:31,954 [125] INFO  umbraco.BasePages.BasePage - [Thread 24] User John Doe (Id: 627) logged in
2015-02-10 16:21:33,775 [125] INFO  umbraco.BasePages.BasePage - [Thread 68] User John Doe (Id: 592) logged in
2015-02-10 16:21:38,251 [125] INFO  umbraco.BasePages.BasePage - [Thread 9] User John Doe (Id: 627) logged in
2015-02-10 16:21:45,114 [125] INFO  umbraco.BasePages.BasePage - [Thread 85] User John Doe (Id: 592) logged in
...