The spec of the computer you've given may, in all honesty, not be "fast" enough to cope with the demands of what you are wanting to do!

Assuming you mean so-called "Full HD" (1920 x 1080p), with the extra burden (on the computer) of Chroma Key, this is possibly beyond the capacity of your CPU and RAM to play back the footage smoothly and/or at the correct speed.  (My computer has an i7 6700K CPU, overclocked to 4.4GHz and 32GB RAM; I've not yet needed to attempt anything on the scale of what you are doing but I suspect it might struggle a little, at least!)

However, have you tried creating/using Proxy files for your HD footage?  These lower-resolution copies allow your computer to more easily process the video (for editing and previewing your work) but any export will use the original full resolution files so ensuring maximum quality is acheived.

Jeff