The painfully slow characterization indicates that something is painfully wrong. The trick is to diagnose.

I would start by looking in the Event Log and also the logs in /portals/_default/logs to see if you can locate information about any problems.

Next, I would look at the webserver, database server, and their resources. What do you have? Versions, memory, etc.

Next look at the browser's developer tools to see if you can see any problems in the console window and look at loading and network performance.

Finally, it is possible that there are some issues remaining from the "not working" version. It is hard to guess what they might be from afar.

Thank you for responding!

I'm not seeing errors in the Event Viewer. I'll see if I can capture some of that today.

Here is a portion of the 2020.07.13.log.resources in /portals/_default/logs from yesterday (635k yesterday). It is full of errors of this sort.

Memory usage on my server is riconsistently running about 98%, CPU beteen 70 and 95%. The only thing running on that machine (it's an Amazon instance) is IIS. We're running IIS version 8, and SQL Server 2014.

2020-07-13 22:37:27,405 [WIN-7SHDS05BLQ4][Thread:12][ERROR] DotNetNuke.Services.Exceptions.Exceptions - System.Data.SqlClient.SqlException (0x80131904): Execution Timeout Expired.  The timeout period elapsed prior to completion of the operation or the server is not responding. ---> System.ComponentModel.Win32Exception (0x80004005): The wait operation timed out
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.SqlDataReader.TryConsumeMetaData()
   at System.Data.SqlClient.SqlDataReader.get_MetaData()
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString, Boolean isInternal, Boolean forDescribeParameterEncryption, Boolean shouldCacheForAlwaysEncrypted)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async, Int32 timeout, Task& task, Boolean asyncWrite, Boolean inRetry, SqlDataReader ds, Boolean describeParameterEncryptionRequest)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, TaskCompletionSource`1 completion, Int32 timeout, Task& task, Boolean& usedCache, Boolean asyncWrite, Boolean inRetry)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
   at DotNetNuke.Data.PetaPoco.PetaPocoExt.ExecuteReader(Database database, String sql, Object[] args)
   at DotNetNuke.Data.SqlDataProvider.ExecuteReader(String procedureName, Object[] commandParameters)
   at DotNetNuke.Data.DataProvider.GetAvailableUsersForIndex(Int32 portalId, DateTime startDate, Int32 startUserId, Int32 numberOfUsers)
   at DotNetNuke.Services.Search.UserIndexer.FindModifiedUsers(Int32 portalId, DateTime startDateLocal, IDictionary`2 searchDocuments, IList`1 profileDefinitions, IList`1& indexedUsers, Int32& startUserId)
   at DotNetNuke.Services.Search.UserIndexer.IndexSearchDocuments(Int32 portalId, ScheduleHistoryItem schedule, DateTime startDateLocal, Action`1 indexer)
ClientConnectionId:e427cdbf-c083-4986-ac64-cb882f518d35
Error Number:-2,State:0,Class:11
 

Did you check if the database is running at it's limits?
Try clearing the logs: https://dnncommunity.org/forums/aft/899

Thank you. I ran the script provided at the link, and it did clear the event log. I'll see if it has any additional good effects.