Regarding linking vs. importing:
I launched QGIS 3.2 and opened the WatercourseLines shapefile that is all lines, from the Australian Hydro data set that became so famous on reddit. It is a 300,000 KB .shp and a 2,171,000 KB .dbf, with about 1.3 million lines in it.
Q starts displaying something right away, as it loads objects from the shapefile (shapefile used to be Q's "native" format) but it took 58 seconds to load the whole thing. So it seems...
(~5s to open the visual - stream network for Australia)
… was just the first five seconds for it to start loading the shapefile, with another 50 seconds or so to go to actually load the thing.
When linking the shapefile, that is, leaving the data in the shapefile, 9 gives you the option of caching locally to get around some of the worst wretchedness of shapefiles, or not caching. With the cache box checked, it took 43 seconds for Manifold to load the whole thing.
With cache on in 9, everything happens almost instantly: pans and zoom box to various areas typically take less than a second. With Q, it's the usual ten seconds or more for redisplay, with 9 being roughly ten times faster.
With cache off, 9 is still very fast. As I mentioned in my post above, despite all the backwardness of shapefiles, the format is not as slow as people think. Q does a good job with it, as expected given that the vector structure of Q for over ten years was based on shapefiles as a native format. 9 also does a good job with shapefiles, but the strategy of 9 is to try to cache and otherwise use the infrastructure of the Radian engine wherever possible to get more speed, even when the data is left in shapefile format.
I guess the bottom line is that if you do an apples-to-apples comparison, leaving the data in the shapefile (your only choice in Q, and using File - Link in 9), 9 is slightly faster to load the data (43 seconds vs. 58 seconds) and much faster (approximately 10x) in using the data thereafter.
If you do an apples to oranges comparison, leaving the data in shapefile in Q and importing it into Manifold .map format with 9, then you have a longer, one-time import with 9 but after that everything is instantaneous. One aspect of that instantaneous opening of .map files is the ability to nest them and to make instant saves. You also get away from the truly stone age limitations of shapefiles on data types.
Last, but not least, with .map you are not limited to the relatively small data sizes allowed in shapefiles.
I'd therefore recommend taking on the one-time import and saving the data in .map. You can always export it quickly enough to .shp should you need to do so for interchange.