I figured out it. It's due to the Auto-Fit Column Width setting. By default, it's enabled. Most files don't have a ton of columns so it's fine, but for 1920 columns, it can really slow things down. When I loaded the file with the setting enabled, it took about 40 seconds. When I tried again with the setting disabled, it took about 1 second.

For now, I may put a band-aid on it with a popup asking if you want to disable the feature when there are a lot of columns. I have some ideas on how to make it more efficient. I'll see what I can do before the full release.