This is classified as an in-game bug.
This happens all the time (10/10).
Open the latest version of Technic Launcher
Ensure that java version is latest (64 bit update 60 java 8).
Allocate 2gb of RAM. I have tried 512mb, 1gb, 4gb, 12gb, they all stutter.
Open Tekkit Legends (takes a minute or so, I have installed on my SSD).
Create world/Play selected world. (Takes several minutes).
Stutters extremely badly.
Sometimes gets better, stuttering once every two seconds, but still unplayable.
Sometimes the game also crashes when opening inventory/chest etc.
I have Java 8 Update 60 installed. I have a powerful enough graphics card (AMD HD 7850, not powerful, but good enough for minecraft). This issue only happens with Tekkit Legends, other modpacks do not suffer from stuttering. I understand that this is in early dev and may still be buggy but I cannot find other people with the same issues.