Both are system, macOS, limitations for QuickLook. 

For the second point, I believe, and I’m sure someone here will correct me, the images and videos that are supported formats (that is handled natively by macOS, don’t seems to have this issue. If the image or video format requires/relies on external app’s quicklook abilities then those will exhibit the same behavior.

Best,
Farhan

On Mar 25, 2018, at 4:08 PM, Christian Rosentreter <karibu@gmx.net> wrote:

* It cuts off files at a certain amount of lines. This wouldn't be so bad by itself, but there
 is zero visual indication that the displayed text is cut off (something like a special coloured
 ellipsis symbol at the end, or something)

* if the system is under load (say something else, e.g. a raytracing application, is using 100%
 CPU) I only get the spinning indicator in the QL window and it never manages to actual display
 the text. It is as the TextMate QL generator runs with such low priority it doesn't get a share
 of CPU by the OS. Other file types (images, even videos) don't seem to share the same problem.