QtLocation's Map uses pixel graphics in low res format which makes it really tiny on High-DPI screens (Retina screens, most newer mobile devices etc). Street names etc get really hard to read. Zooming in doesn't help because it loads the next zoom level tiles which in turn have the fonts tiny again.

Ideally, the maps backends should be converted to use vector based map data, however, that might not be possible in all cases. In such cases, I think scaling the pixel based map tiles to at least have the same size than on low-dpi screens should improve the situation already quite a bit.

Here's some info on how to obtain higher resolution pixel tiles and maybe even vector based tiles for the OSM backend:
https://lists.openstreetmap.org/pipermail/dev/2011-June/022895.html