I'm new to QGIS and work on an air-gapped system where I have access only to QGIS 2.14.9-Essen. I create a WMS layer, which "works" but performance is abysmal. So I enable caching (Settings > Network) with a large disk space.
If I simply pan back and forth by a very small amount (1-2 pixels), I expect it to use the cached files. Instead it apparently fetches new files on each pan, as indicated by the progress bar and a half-second delay. The number of files in the cache area continues to grow with each micro-pan. QGIS is caching ever-more tiles but never using them.
It might help if I understood the file structure. Under the cache area, QGIS created sub-directories data1-data8, and the cached tiles are distributed among those sub-directories, with filenames like .../cache/data6/i8duj3mh.d. What is a *.d file and why the sub-directories?