UE4 Performance issues


I am running UE4 wuth the following hardware configuration

i7 3770 3.4 GHz Processor
8 gb DDR3 RAM
ati radeon HD 5770 1gb DDR5 GPU
Intel mobo DZ77SL

I have recently imported heightmap of Sri Lanka in UE4 , it looks great , but while modifying it , I am facing performance issues , the map is huge , and UE4 frame rate is drastically dropping, and it is becoming very slow.
I haven’t added vegetations yet , which I plan to do , but with vegetations , it is going to get a lot worse.

Please let me know which of these hardware configurations is mainly affecting my performance , and what is your recommendation on this. I haven’t changed the display settings in UE4 , so I guess it is the default setting.