Lots of things:
Single channel memory. (You are halving the potential bandwidth available to the CPU)
Small and slow storage drives (You have limited free space available, but plenty for Virtual Memory)
Limited VRAM (4GB is decent, but still considered entry level, there are many games that exceed 4GB of VRAM)
Textures will be loaded into VRAM, when VRAM is exceeded system memory is used. When system memory is used up, it will then use storage drives and virtual memory. No matter what when a game needs to offload textures to make room for new ones it has to grab the textures from storage from the game's files.
If nothing else, double your system memory. If you can find a really cheap i7-2600, get one.
Start saving for a re-build. Being on a DDR3 system at the end of life of DDR4, not ideal.