PHPStorm background indexing latency during concurrent file edits

Short answer: there is no documented, supported setting to resize PHPStorm's indexing thread pool or to make the UI thread take priority over indexing. The pool size is fixed internally by the IntelliJ platform. Your practical options are to reduce the indexing workload (exclusions, shared indexes), or—at your own risk—override an internal JVM property that may influence the pool size. Power Save Mode remains the only official "UI first" switch, with the trade-offs you already noted.

What is confirmed vs. likely

Confirmed behavior: indexing and inspections run on background threads managed by the platform; the Settings UI exposes no thread-count or priority control; excluding directories and Power Save Mode are documented, supported mechanisms. The event dispatch thread is not given any special priority boost over indexing workers—Swing's default priorities apply.

Likely but unverified: an internal property such as idea.indexing.threads has circulated as a way to cap indexing threads. JetBrains does not document it, its name and effect vary by platform version, and it may be silently ignored in 2023.2+ builds. Treat it as an experiment, not a fix.

Steps for your case

1. Reduce the workload first. In Settings > Directories, mark vendor, node_modules, build output, and generated-code folders as Excluded. This shrinks what concurrent edits force the indexer to revisit, without touching inspections on your actual source.
2. Try the internal property (optional, unsupported). Help > Edit Custom Properties, add idea.indexing.threads=2, restart. A smaller pool leaves more CPU headroom for the UI during simultaneous edits, at the cost of longer total indexing time.
3. Verify the effect. During a re-index (File > Invalidate Caches and Restart), capture Help > Diagnostic Tools > Thread Dump and count indexing-related worker threads to see whether the cap took effect. Compare UI responsiveness during multi-file edits before and after.
4. If on a team, consider Shared Project Indexes (2023.2+), which offload index generation to prebuilt artifacts and reduce local contention substantially.

Caveats

• Thread tuning won't help if the freeze is I/O-bound (slow disk, antivirus scanning the project, network-mounted files). Check that first—concurrent edits often stall on disk, not CPU.
• Custom properties are unsupported and can break or vanish between releases; re-check after every IDE upgrade.
• Search JetBrains YouTrack for "UI freeze during indexing" for your exact build—behavior here changes between releases, and this guidance assumes 2023.2+.

If freezes persist after exclusions and the property change, a CPU snapshot (Help > Diagnostic Tools > Start CPU Usage Profiling) captured during a freeze is the diagnostic that would determine whether indexing threads are actually the culprit.