My GT 740 stays at about 25-30% usage, and all 4 cores of my A10-6800K are at 65-75%, but I get 30 FPS when walking around and dips as low as 14. Render distance is set at 4, and no other graphics options cause a change in FPS.
Playing FTB Infinity Evolved
3GB RAM allocated out of 16Gb total
Have added Optifine to the pack
Windows 10 OS
Vanilla Minecraft, other smaller modpacks, and even SEUS run well
You say SEUS runs fine? How strange... maybe you could update the drivers? Thats all i could think of. This might be malware, run combofix or adwcleaner to check for any malware, those tend to work for me. http://www.bleepingcomputer.com/download/combofix/
Well, Sues runs at 40 FPS, so as good as you could hope from an EVGA GT 740, but maxes out the GPU. And I have ran both AVG and MalwareBytes scans, and they both showed nothing. Drivers are up to date. I just built the computer a month ago if that makes a difference.
Minecraft's debug screen shows RAM usage at around 2GB most the time, and at 2.5GB when loading chunks.
I ran that avg scan a week ago, and have since uninstalled AVG, but this problem started the first time i tried to play infinity almost a month ago. All I have running other than MC is the Curse Voice client (how I am launching the pack) and MSI Afterburner (for usage, temp, and FPS tracking).
I managed to get a steady framerate of 60 (possibly higher if I turned off vsync) and a minimum of 45 by removing by both fastcraft and optifine, then adding COFHTweaks and Better FPS. For those wondering, fastcraft generally causes a minor 2-5fps drop in average frames but helps significantly with lag spikes when loading chunks for me and others I've seen testing it.