Once you have the .csv loaded into QGIS, open the Processing Toolbox and go to Vectory geometry > Geometry by expression.
The 'Input layer' should be your .csv, 'Output geometry type' should be 'Line', and write the following in 'Geometry expression':
make_line(make_point("DS_X", "DS_Y"), make_point("US_X", "US_Y"))
You can speed up CSVs by using the spatial index as stated at QGIS user manual within this chapter.
However, as already stated in comments, using another file format or especially a spatial database like PostGIS can have more effect.
I think @romainbh already gave a better answer, but you can also force QGIS to interpret CSV colums as a special datatype by using a CSV-T.
Therefore create a new textfile, fill it with the column-datatypes of your CSV like this:
You can use Integer, Real, String, Date, Time and DateTime. ...