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For some reason minecraft or a mod is probably not de-allocating ram properly. Check the ram usage when you start and when you get 15 fps (top right in debug screen). These options in 'Advanced Options' in the options tab may help. If you're using 32-bit java switching to 64-bit may also help as well as allocating more ram.
-server -Xms1024M -Xmn512M -Xincgc -XX:UseSSE=4 -XX:+UseCMSCompactAtFullCollection -XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC -XX:+CMSIncrementalMode -XX:+CMSIncrementalPacing -XX:+UseCompressedOops -XX:+AggressiveOpts -XX:+CMSClassUnloadingEnabled
You can also use -XXarallelGCThreads=1 replacing 1 with the number of cores you would like to allocate to ftb. Replace the '' with a colon and a capital 'P'.