I have been playing ftb for awhile now, and I have had this problem but it is getting worse it seems. I have a solid 60 fps always in the game, but it often feels like 15 fps even though it'd 60. This can be quantified by the "lagometer" included in optifine. In the screenshot, the first 1/3 of the lagometer is staring at the ground, the second is looking ahead, and the third 1/3 is looking at the sky(obviously). The white spikes are indicative of when I feel the lag. I am trying to figure out what exactly the lagometer is communicating to us so I can troubleshoot the problem and I figured someone on here might have an idea.
Sorry that I cba to crop out the second display.
Sorry that I cba to crop out the second display.