Hi @Anu,
See if this helps performance.
let
Source = SharePoint.Files("https://properties.sharepoint.com/", [ApiVersion = 15]),
#"Filtered Rows" = Table.SelectRows(Source, each [Folder Path] = "https://properties.sharepoint.com/Shared Documents/Finance/New Kariba Bank Balances/"),
#"Removed Other Columns" = Table.SelectColumns(#"Filtered Rows",{"Content"}),
#"GetWbContent" = Table.AddColumn(#"Removed Other Columns", "Custom", each Excel.Workbook([Content],false)),
#"Removed Other Columns1" = Table.SelectColumns(#"GetWbContent",{"Custom"}),
#"Expanded Custom" = Table.ExpandTableColumn(#"Removed Other Columns1", "Custom", {"Name", "Data", "Item", "Kind", "Hidden"}, {"Custom.Name", "Custom.Data", "Custom.Item", "Custom.Kind", "Custom.Hidden"}),
#"Filtered Rows1" = Table.SelectRows(#"Expanded Custom", each ([Custom.Kind] = "Sheet") and ([Custom.Item] <> "Template")),
#"Removed Other Columns2" = Table.SelectColumns(#"Filtered Rows1",{"Custom.Name", "Custom.Data"}),
#"Renamed Columns" = Table.RenameColumns(#"Removed Other Columns2",{{"Custom.Name", "Name"}, {"Custom.Data", "Data"}}),
#"Added Custom" = Table.AddColumn(#"Renamed Columns", "PromoteHeaders", each Table.PromoteHeaders([Data], [PromoteAllScalars=true])),
#"Added Custom1" = Table.AddColumn(#"Added Custom", "Custom", each Table.Buffer( Table.RemoveColumns([PromoteHeaders], List.Select(Table.ColumnNames([PromoteHeaders]), each Text.StartsWith(_,"Column"))))),
#"Removed Other Columns3" = Table.SelectColumns(#"Added Custom1",{"Name", "Custom"}),
ColumnName = FxDistinctColumnNames(#"Removed Other Columns3",1),
Custom1 = #"Removed Other Columns3",
#"Expanded Custom1" = Table.ExpandTableColumn(Custom1, "Custom", ColumnName, ColumnName),
#"Filtered Rows2" = Table.SelectRows(#"Expanded Custom1", each ([Currency] <> "Currency") and ([Account Name] <> null)),
#"Reordered Columns" = Table.ReorderColumns(#"Filtered Rows2",{"Name", "Bank", "Account Name", "Currency", "Type", "Account Type 2", "Account No. (Kyriba)", "Account Number", "Bot Remarks", "Comments", "10/11/2020", "10/12/2020", "10/13/2020", "10/14/2020", "10/18/2020", "10/21/2020", "10/22/2020", "10/23/2020", "10/24/2020", "10/25/2020", "10/26/2020", "10/27/2020", "10/28/2020", "10/29/2020", "10/30/2020", "10/31/2020", "11/1/2020", "11/2/2020", "11/3/2020", "11/4/2020", "11/5/2020", "11/6/2020", "11/7/2020", "11/8/2020", "11/9/2020", "11/10/2020", "11/11/2020", "11/12/2020", "11/13/2020", "11/14/2020", "11/15/2020"}),
#"Unpivoted Other Columns" = Table.UnpivotOtherColumns(#"Reordered Columns", {"Bank", "Name", "Account Name", "Currency", "Type", "Account Type 2", "Account No. (Kyriba)", "Account Number"}, "Attribute", "Value")
in
#"Unpivoted Other Columns"