The fact that you're using shared hosting may be the reason of the slowdowns. You don't know how many resources the other websites on your server are using. For a website of that size, I'd recommend to get a dedicated server instead of some shared hosting plan. For more information, see http://www.webhostingtalk.com
I'd also recommend to disable GZip as that adds some extra processing on both server and client side; you'll only have to enable it if you are using too much bandwidth (which would be another reason to get your own box).
In my opinion, the tweaks in the topic you linked to are pretty straight-forward.. that is, if you have some experience with modifying phpBB.
The phpbb_search_wordmatch table is one of the largest tables in a phpBB database, that's normal