QGIS very slow to display symbols (~1100 entities)

Question

I am working on a project used for data capture on fields, through Qfield.

Today, with around 1100 entities, QGIS takes a lot of time to display the information, even if I use a single symbol.

Could you tell me what is slowing down QGIS ?

I have a few leads:

• I'm using many relations (in this project, 7 for each of the 3 layers).
• I'm referring to the relations to display labels (but disabling labels doesn't change anything)
• I'm using a virtual field (I have read that these fields can slow down QGIS)
• Is the CRS can have an impact ?
• Should I create Spatial index ?

I'm only using Shapefile. I also deactivate the option "Show feature count" and I removed all the variables to control shapes and colors.

On stackexchange.com, I found several discussions with a similar subject like: Link1 Link2

I add a link to download a simplified project if you want to check. At least opening it and telling me if you have the same issue. Test_project

My computer is working under Windows 7, RAM: 16Go, CPU: Intel Core i7-7700HQ 2,80GHz.

EDIT:

I updated my project as advised. No more virtual field, which make the project faster. But now, without virtual field, I cannot get the information from relation layer(s).

I created a normal field for which I apply default value on update and I activated data dependencies on the relation layers. This field is only updated if I modify its layer.

EDIT 2: I uploaded a second test project with the modifications. Test project 2

Answer 1

The reason your layer is slow is the virtual field. This is your expression, which is tied to the symbology of your layer. So every feature is running this every time the renderer is triggered.

CASE
    WHEN "COMPART" = 1 THEN 
        CASE 
            WHEN Attribute(Get_feature('[T]_DATA_Relation_Habitats','id',"id"),'Validite') = true THEN
                 represent_value("COMPART") || ' - '  || Attribute(Get_feature('[T]_DATA_Relation_Habitats','id',"id"),'Habitat')  ||  ' - '  || Replace("date",left("date",strpos("date",'-')+1),'')  ||  ' (V)'
            ELSE
                represent_value("COMPART") || ' - '  || Attribute(Get_feature('[T]_DATA_Relation_Habitats','id',"id"),'Habitat')  ||  ' - '  || Replace("date",left("date",strpos("date",'-')+1),'')  || ' (X)'
        END
    
    WHEN "COMPART" = 2 THEN  'Soil survey: ' || Attribute(Get_feature('[T]_DATA_Relation_Sondages','id',"id"),'id_sondage') || ' - '  || '[' || Attribute(Get_feature('[T]_DATA_Relation_Sondages','id',"id"),'Res_sond')  || ']' ||  ' - '  || Replace("date",left("date",strpos("date",'-')+1),'')

    WHEN "COMPART" >= 3 AND "COMPART" <= 9 THEN 
        CASE 
            WHEN Attribute(Get_feature('[T]_DATA_Relation_Species','id',"id"),'Validite') = true THEN
                represent_value("COMPART") || ' - '  || Attribute(Get_feature('[T]_DATA_Relation_Species','id',"id"),'Code_esp')   ||  ' - '  || Replace("date",left("date",strpos("date",'-')+1),'')  || ' (V)'
            ELSE
                represent_value("COMPART") || ' - '  || Attribute(Get_feature('[T]_DATA_Relation_Species','id',"id"),'Code_esp')   ||  ' - '  || Replace("date",left("date",strpos("date",'-')+1),'')  || ' (X)'
        END 
    WHEN "COMPART" = 10 THEN 
        CASE 
            WHEN Attribute(Get_feature('[T]_DATA_Relation_Wetlands','id',"id"),'Validite') = true THEN
                represent_value("COMPART") || ' - '  || Attribute(Get_feature('[T]_DATA_Relation_Wetlands','id',"id"),'Nom_ZH')   || ' - '  || Replace("date",left("date",strpos("date",'-')+1),'')  ||  ' (V)'
            ELSE
                represent_value("COMPART") || ' - '  || Attribute(Get_feature('[T]_DATA_Relation_Wetlands','id',"id"),'Nom_ZH')   ||  ' - '  ||Replace("date",left("date",strpos("date",'-')+1),'')  ||  ' (X)'
        END     
    ELSE
represent_value("COMPART") || ' - '  || "Other"  ||  ' - '  || Replace("date",left("date",strpos("date",'-')+1),'')
END

You'll need to save these values into non-virtual field, or find a way to condense the logic.