I'm using QGIS for a while. In my opinion the SAGA kriging tool (processing toolbox) is very slow. Is that an QGIS issue? I tried to interpolate a point layer with 300.000 points (I know it is big) and it takes round about 1 hour to get the interpolated result. In SAGA it works much more faster.
A lot depends on your desired grid size output. Kriging itself is generally a computationally expensive process due to the numerous calculations that take place. In qgis 2.18 I regularly use the SAGA ordinary kriging algorithm as part of a model; outputting on a 3m grid size I can process 100000+ points in under 15 minutes. I can see how 300000 points would take a long time. I've researched other methods, software and tools to speed up the process; to date I haven't found anything to speed up the process.
In my current workflow I just setup models to do the required tasks and go do other things while the model is processing.
