Direct template modifications should not have an overall impact on the speed of your forum, unless the page sizes are overly large themselves or the server you're on is bogged down or overloaded.
If you're planning for growth and you're in a shared hosting environment now, VPS (Virtual Private Server) would be the next step up for you and still remain a step down from a dedicated server (as well as remain a bit cheaper).
Possibly check ASmallOrange.com or related companies for VPS packages.
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