There are many possible reasons for this, but most have to do with memory or session database size.
See the documentation on MINIVEND ADMINISTRATION and learn how to expire your session database. If it gets many megabytes large, then accessing a key will get slow.
Second, if your machine is memory-poor you will find that MiniVend gets swapped to disk. Unless your system is very fast, this will greatly hurt performance. MiniVend works best on a machine that rarely if ever swaps to disk.
Third, this often has nothing to do with MiniVend at all but has to do with your HTTP server. Any long-running daemon has the potential for a memory leak -- try stopping and starting your HTTP server and seeing what happens to performance.
On Windows, you will want to stop and re-start your MiniVend server at
least once per day. You can use a system agent to automate it -- you will
want to stop the server to expire the session database anyway. You need to
do this because MiniVend requires the Unix-specific fork()
call to spawn multi-tasked servers, and without multi-tasked servers there
is a strong possibility of a memory leak -- especially if you use embedded
Perl or conditional tags.