I doubt that only changing a literal text string into a variable will have the effect of speeding up a load by a factor 15. What can help is not doing anything that prevents a qvd from loading in optimized mode. See this blog post for more information: https://www.quickintelligence.co.uk/qlikview-optimised-qvd-loads/
Both of your example-statements does the same and will have the same run-time (the differences within the path aren't important for the run-times unless one is loading from a fast storage and other from a slow network). What do you noticed is difference between a normal qvd-load and an optimized qvd-load. By an optimized load happens no (real) processing of the data - they will simply transferred within the RAM. A qvd is nearly exactly a 1:1 image from RAM. In this way is only allowed to rename fields and to use a where-exists-clause with a single parameter - all other transformings will need some processing and therefore prevent the optimized load.