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Without knowing exactly which forum you mean - and without knowing much of forum software - I would say that there are some good reasons for the number of SQL queries.
A forum, what does it have to fetch in the database? User details if logged in, configuration settings, a list of forums available to the user, number of posts in each forum... that kind of thing.
What does XOOPS have to fetch in the database? User details if logged in, configuration settings, modules installed, blocks, information to show in the blocks, information about the module, you are in, information to be displayed in the main content area, permission checks...
Not everything in XOOPS is optimised towards few SQL queries and some work can still be done there, but don't forget that XOOPS has so much more settings and information than a forum.