Why does it take longer and longer to export my maps as png in a loop?

Question

And how can I improve my performance? I wrote a simple python script to export a few hundred maps:

Open a MXD, add a layer, apply a definition query, apply a symbology, export to PNG.

The definition query is in the loop. Everything else is always the same. And there are always 153 features selected in the FC. In the first three minutes between 10 and 20 pngs are produced. After that only 2. How can I prevent this and improve the performence? There are hundreds of pngs waiting.

import arcpy,os

field1 = "Field1"
mxd = r"X:\GIS\test.mxd"
mxd2 = arcpy.mapping.MapDocument(mxd)
df = arcpy.mapping.ListDataFrames(mxd2)[0]
shp = r"X:\GIS\TestGDB\test"
dbf = r"X:\GIS\testdata.dbf"
elementlist = ["value1","value2",...] #here are a few hundret elements in the list
seasonlist = ["spring","summer","autumn","winter"]
for element in elementlist:
    for index,season in enumerate(seasonlist):#[0:1]:
        where_clause = '"%s" = %s' % (str(field1),str("'"+element+"'"))

        layer = shp.split(os.sep)[-1]
        layername = layer
        arcpy.MakeFeatureLayer_management(shp,layername)
        layer = arcpy.mapping.Layer(layer)
        layer.name = layername + "_" + season

        layer2 = arcpy.mapping.TableView(dbf)
        layername2 = layer2.name + "_" + season

        layer2.definitionQuery = where_clause

        arcpy.mapping.AddTableView(df,layer2)
        arcpy.AddJoin_management(layer,in_field,layer2,join_field,"KEEP_ALL")

        expression = "!" + symbologyfield + "!"
        arcpy.CalculateField_management(layer,valuefield,expression,"PYTHON")

        arcpy.RemoveJoin_management(layer)
        #arcpy.UpdateLayer(df, update_layer, source_layer, {symbology_only})

        arcpy.ApplySymbologyFromLayer_management(layer,layerfile)
        arcpy.mapping.AddLayer(df,layer,"TOP")

        layer.visible = True

        for element2 in arcpy.mapping.ListLayoutElements(mxd2,"TEXT_ELEMENT"):
            if element2.name == textfield1:
                element2.text = str(element)
            if element2.name == textfield2:
                element2.text = str(season)


        arcpy.RefreshActiveView()
        arcpy.RefreshTOC()

        arcpy.mapping.ExportToPNG(mxd2, output + os.sep + str(element) + "_" + str(season) + r".png",resolution=printresulution)

Answer 1

There should be no need to include:

    arcpy.RefreshActiveView()
    arcpy.RefreshTOC()

unless you are running this from within ArcMap and want to see your maps displayed in that application before exporting them.

At the moment you are only presenting a copy/paste of a loop from your code rather than a code snippet that starts with import arcpy and works up to where you are stuck.

Writing code snippets to get quicker answers? provides advice on what a code snippet for an ArcPy question should look like.