Excel is built to handle large amounts of data, but certain spreadsheet habits push it well beyond what it can comfortably manage in memory, and the result is exactly the sluggishness and warnings many Mac users run into.
What actually drives Excel's memory use up
Every open workbook, every worksheet within it, and every formula that references other cells is held in memory while Excel is running, so it can recalculate instantly whenever something changes. This is normally efficient, but a few specific habits scale that memory use dramatically. According to Microsoft's own documentation on Excel recalculation, volatile functions (formulas like NOW, TODAY, RAND, and OFFSET that recalculate every single time anything on the sheet changes, rather than only when their own inputs change) force Excel to redo far more work than a typical formula, and a sheet with many of them can slow recalculation noticeably. Formulas that reference entire columns (like a SUM covering column A in full rather than a specific range) make Excel evaluate far more cells than the range you actually care about, even when most of that column is empty. And conditional formatting rules applied across large ranges add ongoing evaluation overhead every time the sheet recalculates or redraws.
Why multiple large workbooks compound the problem
Excel keeps every open workbook fully in memory for as long as it's open, even ones you're not actively looking at. If you regularly work with several large spreadsheets open simultaneously, perhaps cross-referencing data between them, Excel is holding all of their formulas, formatting, and cached values in memory at once, and the combined load is what typically pushes things from "a bit slow" into genuinely sluggish or memory-warning territory.
External data connections and add-ins add their own overhead
Workbooks connected to external data sources, a database query, a Power Query import, a live feed, hold additional memory for that connection and its cached results on top of the spreadsheet's own content. And third-party Excel add-ins, if you have any installed, run continuously alongside Excel and can carry their own memory footprint regardless of whether you're actively using their features in your current session.
What actually helps
Replace whole-column references with specific ranges. Changing a formula from summing all of column A to summing just the rows you actually have data in (A2:A5000, for instance) can meaningfully reduce recalculation load on a large sheet, since Excel no longer evaluates thousands of empty cells for no reason.
Minimize volatile functions where you can. If a formula doesn't genuinely need to recalculate on every single change, a non-volatile alternative (or simply calculating a value once and pasting it as a static number) avoids the repeated recalculation cost entirely.
Close workbooks you're not actively using. Rather than keeping five spreadsheets open across your workday, closing the ones you're not currently referencing frees up whatever memory they were holding for their formulas and formatting.
Review your conditional formatting rules on large sheets. If a sheet has accumulated many overlapping conditional formatting rules over time, consolidating or removing redundant ones reduces the ongoing evaluation cost on every recalculation.
Restart Excel periodically during long work sessions. Since cached data from external connections and general application state can accumulate over a long session, closing and reopening Excel clears that out and gives you a clean baseline.
Switching calculation to manual for genuinely large workbooks
Excel recalculates automatically by default, meaning every time you change a cell, it re-runs every formula that could be affected, which is convenient for smaller sheets but can turn a single edit into a multi-second wait on a sufficiently large or formula-heavy workbook. Switching calculation mode to manual (found under the Formulas menu on Excel for Mac) stops that automatic recalculation, letting you make several changes in a row and then trigger a single recalculation deliberately when you're ready. This doesn't reduce how much memory the workbook itself needs, but it does reduce how often Excel is actively working through that memory-heavy recalculation process, which is often what's actually causing the sluggish, unresponsive feeling while you're editing.
When it's a genuinely oversized file, not just an inefficient one
Some spreadsheets are simply larger than Excel is comfortable holding in memory regardless of how well-optimized the formulas are, particularly ones that have accumulated years of historical data, many years of monthly tabs, or large amounts of pasted-in data from other systems. In that situation, the more durable fix is usually splitting the workbook into smaller, purpose-specific files, or moving the underlying data into a proper database and pulling only what you need into Excel via a query, rather than continuing to optimize formulas within an inherently oversized single file.
Where a memory-relief tool fits in
None of this replaces good spreadsheet hygiene, that's the actual fix for a genuinely oversized or inefficient workbook. Where Shiny helps is alongside it: freeing up memory tied up by everything else running on your Mac (other apps, background helpers, browser tabs) so Excel has more available headroom to work with when you're processing a large, legitimately demanding spreadsheet.
For a related productivity-app pattern, see Mail on Mac using too much memory, which covers a similar accumulated-data memory pattern in a different app.