Jakub,
The editor in 4.3 introduces the extensive syntax checking in the editor. As you edit, EG is simultaneously updating the checking and color coding. If your PC has limited memory or CPU speed, you will see a lag. I'm not sure if there is a way to run off the syntax checking. You might try breaking the program into pieces and running in sequence or using %INCLUDE.
I do big programs in EG 5.1 (Win 7 Pro, 4 GB, 4 processor) and have no lag in the programming.
Doc Muhlbaier