I imported a 15MB text file (10,000 lines) using Read-File-Into-Node. It worked, but it took a long time. Scrolling or jumping within the file also took a very long time. Jumping from the end to the start took several seconds. A search from the start to a phrase at the end took about 5 seconds or so. The cursor couldn't track as I marked a phrase, but after several seconds the phrase became highlighted.
Also, after moving away from this tab to another Leo file, moving back to this one took about 15 seconds before it displayed. And operations in other Leo outlines open at the same time was sluggish.
And I have a pretty fast computer.
For comparison, Notepad++ on Windows loaded the same file in under 1/2 second.
I presume this behavior is a limitation of the underlying QT5 implementation, though I don't see why sluggishness bled into other tabs.