Regarding absolute speed - for most thing VB will be much quicker as it is supposedly compiled whereas HN is interpeted. I say compiled because with .NET there is a virtual machine that slows down the code appreciably - 8 times slower than Delphi doing maths.
In practical terms HN is quick enough for most things - I've attached a project I use every day to analyse our web log files. On this 500MHz G4 it can do one loop over a 20000 line file in less than a second.
If you need to make HN faster it can run RBscript (REALbasic) and this has a rich set of string commands and is very fast. There are some demo projects on our web site showing how to use RBscript. With RBscript for text processing, you pass it a variable, run the script and it returns the results in another variable.
Regarding speed, perhaps you could use the project I sent you to do some tests before committing yourself to either HN or VB.
HN does have some nice string handling and replacement functions but if you have to use lots of statements within loops it can be slow. RBscript within HN doesn't have this problem as its compiled but its not as user friendly as HN script.