What's actually happening
When you copy rows from one file into another, you're doing what databases call a join — matching records by a shared column (account number, job number, customer ID) and pulling across the details from the second file.
The reason it takes so long is that Excel does not do this automatically in the grid. Every row requires its own filter-and-read step. If either file changes shape — new columns, a different sort order, a heading renamed — the whole process breaks and needs rebuilding.
This is a normal problem with normal spreadsheets and there is a built-in fix that does not require formulas or technical knowledge.
The Power Query merge
Power Query is built into Excel on Windows (Excel 2016 and later). It connects your two files and joins them on the shared column so you never open both files manually again.
Step 1 — connect to File 1. Open Excel and look for Get Data under the Data tab, then From File, then From Workbook — menu names move between versions, so check your version if it looks different. Pick your account list (File 1). A preview opens. Select the sheet and click Transform Data.
Step 2 — connect to File 2. In the Power Query Editor ribbon, click New Source, then File, then From Workbook. Pick your records file (File 2). Select the sheet and click OK.
Step 3 — merge the two. With File 1's query open, click Home, then Merge Queries. In the dialog, pick File 2 from the dropdown. Click the shared column (Account Number) in File 1, then click the matching column in File 2. Leave the join type as Left Outer — this keeps every row from File 1 even when File 2 has no matching entry. Click OK.
Step 4 — expand the columns you need. A new column appears with an expand icon in the header. Click it. Tick the columns you want (description, date, category) and untick the rest. Click OK.
Step 5 — close and load. Click Home, then Close and Load. Excel builds a sheet with the completed report — all the matching done automatically.
The root-cause wording
If you classify descriptions into standard phrases (your own standard wording for the causes you see most), add a third table: two columns — keywords in one, your standard wording in the other. Merge this as a second step in the same query. The classification happens inside the query, not by reading each row.
Refreshing the next morning
Open the report file. Right-click anywhere in the output table. Click Refresh. The query re-reads both source files and regenerates the output. The daily work becomes two clicks.
When this will not work
Power Query reads files from a fixed path. If your files live on a shared network drive that changes its drive letter between machines, or if the filenames include today's date, you need one extra step: store the path in a named cell and reference it in the query settings. This is a single setup session with a bookkeeper or a support person who knows Excel.