Well,
I don't believe FB is too much resource hungry. The number of queries are high (on large websites) but they are pretty fast and repeated queries (unfortunately, will be revised)
Currently at our forum there are : 7568 users, 24000 posts and 60-150 posts a day and overall 5M hits. The processor is usually under 10% and the load is less than 1. Needless to say that we have several sites on the same server.
Also, in another forum, we have 400+k posts and 40+k members (there is the origin of Fireboard

) and daily hit of more than 1M only on the forum, and the forum that handles is Fireboard. It has some load (around 4, mysql 30%-60%), but with the risk of using another forum (like SMF nightmares) Fireboard is a bliss.
I would suggest to install the latest FB (or upgrade to it) and check the indexes of the database (you can install a clean one on your local host and compare the databases), also in addition you can disable Stats plugin and save a lot of resources.