yes, enabling php could hurt performance to a degree but lets break this down. (hopefully there are no errors in my math =) )
61,440,000k a month / 30 days in a month = 2,048,000k a day.
2,048,000k a day / 24 hours in a day = 85,333k a hour.
85,333k a hour / 60 minutes in an hour = 1,422k a minute.
1,422k a minute / 60 seconds in an minute = 23k of bandwidth is being used per second.
so with decent hardward and light database access (or database on another server) your site should be able to handle .htm & .html files as php with no problems.