Sometimes when you want to load data from a WFS web service, QGIS takes ages to load it or even seems to be unresponsive. Then there are some small settings you can change to try to fix this problem.
A reason this can happen is that some WFS services offer huge amounts of data. Some WFS service limit themself and will for example give you a maximum of 1.000 features. Those web services are probably not the ones that will take ages to load. But there are also other WFS services that don’t limit themself and sometimes they contain millions of features. Those web services will probably take a long time to load and it is also for those WFS services that we can try and fix the problem as a QGIS user.
So the main trick if the web service doesn’t limit itself with the amount of data it sends to us, is that we limit that web service. There are two ways we can do this and I will describe both methods. You can use one of them or combine them.
Only request features overlapping the view extent
Important thing to do before applying the setting below, is that you zoom in to the area you want get data for from the WFS web service.
We can find the “Only request features overlapping the view extent” setting when we go to the Data Source Manager in QGIS and there go to “WFS / OGC API features”. After we connected to the WFS service, you can select which layer you want to add to QGIS, but before you do that, check the “Only request features overlapping the view extent” box below the list of layers.
When this setting is checked, QGIS will only request data from the WFS service that is within the area you have currently in view. If you zoomed in sufficiently this will mean it won’t try to download all data from the web service, but only a limited subset.
Maximum number of features
The other setting you can change to limit the number of features, can be found within the WFS Connection settings. One of the WFS Options is “Maximum number of features” and if you enter a not too big number in that, it will only request that amount of features from the WFS service. Important to know is that if you set that setting to for example 200, that the WFS service will send the first 200 records to QGIS. If you don’t use the “Only request features overlapping the view extent” setting that was explained above, this will probably mean that some (or all) of those 200 features will be out of view and not within the area you are interested in. So this setting can best be combined with the previously explained setting.
As an example I set it this setting to 200, but depending on how fast the web service is, you can increase this number if data loads fast or make it smaller if the data still loads slow.
Anything else?
Well, there can be many reasons why data from a WFS service loads very slow. The tricks above will only help if the WFS service tries to send too much data to QGIS to handle. If anything else is the cause the loading of data is slow, you will have to look further for a solution.