I've noticed, since April 2025, Power BI Desktop opens a lot slower than usual. Yesterday, I noticed that a simple change to a (thin) report bloated the filesize from 83 kb to 2442 kb. Today, speaking extensively with Copilot, I realize the following: bloat is caused by two built-in themes: Bloom.json and Tidal.json. I can strip them both down to 1 kb a piece by Copilot, and copy them in. Now, opening the pbip-based report is fast! However, saving the very same report as pbix once again returns the bloated theme files!! In short, I have hit a roadblock here, and I smell a bug in a recent release of Power BI Desktop (I run "Version: 2.144.878.0 64-bit (juni 2025)") which causes too long opening times in general, and too big files for some edge cases - one of them triggered by myself yesterday. If only I keep the pbip structure, I have no problem. The problem arise in the pbix save moment - and the original themes are super-inefficient in two specific cases as mentioned above.
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