Expression variables are calculated too often

[Joonalai]

What is the bug or the crash?

When trying to figure out why QGIS gets frozen and really slow with more than 200 layers, I noticed expression variables are calculated really often.

Steps to reproduce the issue

To reproduce the issue it's easiest to add log line to QgsExpressionContextUtils::layerScope method and then do following steps:

1. Add layer to the project
2. Digitize one feature
3. Move map canvas
4. Calculate lines on the log panel

These simple steps produce 85 lines for memory polygon layer, 123 lines for gpkg polygon layer and 226 lines for PostGIS layer.

Versions

Version details

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QGIS version	3.40.2-Bratislava
QGIS code revision	14826ca
Libraries
Qt version	5.15.13
Python version	3.12.3
GDAL/OGR version	3.8.4
PROJ version	9.4.0
EPSG Registry database version	v11.004 (2024-02-24)
GEOS version	3.12.1-CAPI-1.18.1
SQLite version	3.45.1
PostgreSQL client version	16.6 (Ubuntu 16.6-0ubuntu0.24.04.1)
SpatiaLite version	5.1.0
QWT version	6.1.4
QScintilla2 version	2.14.1
OS version	Ubuntu 24.04.1 LTS
Active Python plugins
db_manager	0.1.20
MetaSearch	0.3.6
processing	2.12.99
grassprovider	2.12.99

QGIS version 3.40.2-Bratislava QGIS code revision [14826ca](https://github.com/qgis/QGIS/commit/14826ca1e4a)

Libraries
Qt version
5.15.13
Python version
3.12.3
GDAL/OGR version
3.8.4
PROJ version
9.4.0
EPSG Registry database version
v11.004 (2024-02-24)
GEOS version
3.12.1-CAPI-1.18.1
SQLite version
3.45.1
PostgreSQL client version
16.6 (Ubuntu 16.6-0ubuntu0.24.04.1)
SpatiaLite version
5.1.0
QWT version
6.1.4
QScintilla2 version
2.14.1
OS version
Ubuntu 24.04.1 LTS

Active Python plugins
db_manager
0.1.20
MetaSearch
0.3.6
processing
2.12.99
grassprovider
2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

Maybe some kind of caching should be used to limit amount of times variables are calculated?