I dont necessarily think a buff in processing time is the right move due to the tools already in place (the ability to process your whole vehicle in one go).
The problem seems to mainly stem from the fact that legal runs aren't that far off from Illegal runs profit wise with WAY less risk, a more reasonable route could be to increase the price of ALL illegal runs to where the reward outweighs the risk as again, in its current state legal runs are just safer and easier money in comparison.
I also dont think nerfing legal runs profit wise is the right move (besides a few exceptions).
One could also do a little revamp of locations, make Legal runs a little less of a "straight drive" whilst also making illegal runs not necessarily a shorter drive but a easier one. (Meth being a prime example of a discouraging drive)