Let's take a look at the project, per the instructions above. That's the best way to get a handle on issues.
If source files are very high-def, lengthy, or H265 (HEVC) encoded, consider Proxy Editing, as described in the 'sticky' post above.
This common NLE practice uses smaller files during editing, substantially freeing up resources and cache load. When editing is complete, the original files are substituted, if necessary.
A lower export resolution, say HD 1080, might save time and look very good on PC monitors.