I suspect that Jimmy has something there, though there is no UI "beta" indication on the 6.20 being run here.
Remember, too, that video editing is resource intensive. Some general thoughts...
Use video cards or integrated graphics that are up to the task - with current video drivers
Confirm that disk free space is adequate
Run a recent, fast processor with ample RAM (8 minimum would be wise)
When exporting, shut down other programs
Pray