Hello, bjmjpl!
The capture card you're using, I've not come across that brand before. Nonetheless, here's my two cents worth(copper coins are worthless in Australia these days, though! LOL). I'm guessing that you're doing a bulk capture, i.e., capturing the entire content of a tape in one "gulp" (burp!) Sorry, just adding a bit of levity there! but seriously, try just taking half-to-one-hour at a time, processing it, tuck it away in a file folder, then take the next half-to-one-hour's worth of content, working, piecemeal, as it were, and it may make your computer's job a bit easier. Capturing and editing video is resource-heavy, it puts a huge burdon on the computer to process all that content. I suggested half-to-one-hour, but more practically, watch the capture screen for any "breaks" in content and end the capture at that point. You can then "play around" with that fresh capture(edit it, clean it up) or just store the capture under a tentative title and edit it later. Basically, it's better to have several small easy-to-handle "grabs" than one large resource-hungry file.
Cheers!
Neil.