This is unfortunately unavoidable, as it relates to the high amount of background simulation TFC does, and the large amount of blocks loaded (the sea level is more than twice as high as in vanilla).
You might be able to improve performance with with additional java parameters: -XX:AggressiveOpts -XX:UseSSE=3 -XX:MaxPermSize=128
The FTB Launcher already starts with AggressiveOpts enabled by default (can be seen under snooper settings ingame), but if you are using a regular Minecraft install this can help a lot (it got me around 15% less CPU load when I slapped it on my Mindcrack v6 server). UseSSE needs to be adjusted to the best version your CPU is capable of; you can find out what that is by using the freeware tool CPU-Z to read out your processor's specs. MaxPermSize increases the amount of memory the java VM is allowed to reserve for itself; default is 64. It may help, or it may do nothing at all - depends on the situation.