Hi, I have been using QGIS and SAGA for around 10 years.
QGIS is a great application; recently I have been using it to answer complex probem-sets with big/huge datasets and I find it is pushing the application (not my machines) to its limit. The GUI version requires a lot more development to fully embrace parallel processing.
My current challenge is finding and trying alternative workarounds to run tasks quickly (e.g. using PYQGIS scripts) and ideally in parallel using multiple processors (by running PYQGIS outside the GUI, inside the OS shell). Developing these can be frustrating and time consuming due to the lack of good examples in the official documentation and online; especially with the changeover to QGIS3. I have always found GIS Stackexchange to be a great forum to share ideas and get help with problems like these.