What is the performance of the pages being loaded? You can see by going into debug mode from the link in the footer. Are some pages taking a fairly long time to load?
I ask because it's fairly normal for some MyBB pages to have a large number of queries, but the queries are usually very fast. I have a very old VPS with limited hardware and they run in an average of about ~1/1000 second each.
Most likely, either one particular query is being very slow (which happens more often than would be expected), or they are all unusually slow. If every query is slow, it might be a hosting/server issue. If it's just one query, it's easier to address and to fix that one query.
25 queries itself isn't a huge number of queries necessarily, so perhaps looking a little deeper into them might be the answer performance-wise. What does the debug page look like on various pages for you?