Editing software

I currently use shotcut, a free video editor. All the free ones I've tried, the preview video is laggy. Anyone recommend a good, free editing software with smooth preview?

The preview workspace is usually due to the processing power of your computer. The better the computer, the smoother the output of the preview. However, this shouldn't be a problem during export.

What sort of computer do you have 16ratnettfi?
As above once its exported it shouldn't affect the final result when exported.

Google Earth Studio is free but it can take awhile to render.

Blackmagic Davinci Resolve? It's a NLE and Colour Grading programme but works quite well.

Just standard dell computer tower.

Dell have made thousands, maybe millions of computers since their conception in 1984, you need to be a bit more specific.
Somewhere on your Dell computer case will be a service tag, often on the back, bottom or side of the case. What's your service tag?

Dell Optilex 780.

Intel Core 2 / Pentium / Celeriac with up to 8 GB of RAM seems to be the base specs. No external graphics card.

a bit "past it" then. I suppose it takes all day to export a video as well? But does the exported video play okay? If that's the case then I wouldn't worry too much about what the preview shows because you're the only one who sees that anyway.

Exported video is ok but it takes so long to edit a short piece of footage because you can't see what bit your editing as it's so laggy.

Best thing I will say is grab the newest graphics driver from Dell's website and see if that fixes the problem.
If it doesn't, then you may have to consider spending some money to upgrade or replace the computer if the preview issue happens on all video editing programs. Unfortunately video editing requires decent hardware, but that being said I have been able to do it on my previous laptop so...