u/DeathMaillet

I’ve been doing a fairly deep performance pass on an Excel/VBA scheduling engine, and I think I’ve reached an interesting architectural limit.

The current stress test is around 1,200 tasks, ~2,000 business shapes and ~1,180 dependency links.

A lot has already been optimized:

• scheduling core and analytics run from compiled/indexed structures
• the watcher is almost free
• rendering is local/incremental
• ~3,100 dependency Shapes were replaced with a single SVG layer

Despite that, a very small local change can still take ~10–13s, while a heavily propagated change can take 35–90s.

The surprising part is that COM writes are no longer the main problem.

On one propagated case, 661 shapes were updated with 3,811 COM property writes, but those writes only took ~1.2s.

The real cost is now mostly before the writes:

• a global O(n²) hierarchy pass still runs before the local filter and costs ~8s by itself
• timeline geometry is recalculated thousands of times and repeatedly reads .Left and .Width from worksheet cells, creating thousands of COM reads
• style-only updates such as CP/LP still pass through geometry-building code
• a fixed “change set too large” threshold forces a broad render path once more than 400 IDs change

My next step is probably to make the renderer much more transactional:

• precompute hierarchy in O(n)
• preload timeline geometry into arrays
• render directly from changed IDs
• remove the fixed fallback threshold
• create a true style-only path for CP/LP
• keep the current renderer as a fallback

What I’m curious about is this:

For people who have pushed Excel/VBA renderers hard, where did you find the real practical limit?

At this stage I’m not really looking for the usual “use arrays instead of cells” advice — that part is already done. I’m more interested in projection/cache structures, COM-read avoidance, or architectural tricks that gave you a real order-of-magnitude improvement.

When I started my last project, I saw VBA as a slightly obscure way to automate repetitive Excel work and push formulas a bit further.

Then every time I thought I had reached its limit, I found another door.

What began as a few scheduling calculations gradually turned into dependency graphs, critical-path analysis, incremental recalculation, automated tests and an interactive Gantt built with Excel shapes.

The real turning point was when I stopped treating Excel as the application and started treating it as the interface. The workbook handles inputs and outputs, while most of the actual logic runs in memory through arrays, dictionaries and separate modules.

VBA is still old, awkward and occasionally infuriating. But I expected it to do a little more than Excel formulas.

It turned out it could do a lot more.

I’m curious: what did you build that made you realise VBA was more capable than you initially thought?

Hi everyone,

Over the past months I've been building an open-source scheduling engine entirely in Excel/VBA.

The original idea was simple.

Throughout my career, I've worked with teams that already managed their schedules in Excel but either didn't have Microsoft Project licenses, didn't need a full enterprise scheduling tool, or simply preferred staying in an environment that everyone already knew.

I wanted to see how far a native Excel planning engine could realistically go.

Today the project includes:

• Dependency-based scheduling (FS / SS / FF + lags)
• Interactive Gantt chart
• Critical Path analysis
• Total & Free Float
• Incremental recalculation
• Scenario simulation
• Planning diagnostics
• S-Curve
• Fully documented architecture

The project has just reached version 1.0.1 and is completely open source.

GitHub:

https://github.com/TMailletFR/ProjectEngine

A few questions immediately come to mind:

• Could you see yourself using something like this in your organization?
• If not, what would stop you?
• Which scheduling features would you consider essential before trusting it on a real project?
• If you already manage schedules in Excel, what are the biggest pain points you've run into?

I'd really appreciate any feedback, whether it's about missing functionality, usability, or simply whether you think Excel is the wrong platform for this kind of tool.

Thanks!

Gantt

S-Curve