Hello,
I had very bad constant lag spikes like every 3 seconds (unplayable), I have looked in debug screen and It was when memory was GC-ed. Anyway I had pretty good fps 60-100 but It was unplayable so many lags.
I have disabled these mods which are almost not necessary:
-AromaBackup
-Hats
-HatStand
-JourneyMap
-Opis(should be disabled in default)
In game disabled mipmap and antistrophic filtering, also clouds off.
I have also added these mods: voxelmap (+liteloader) and fastcraft now It runs very good. Just wanted to share and hope this helps to someone!
My system:
Windows 8.1 64 bit, Intel Pentium CPU B960 @ 2.2GHz, 8 GB RAM, AMD Radeon HD 7610M