Whenever a long string is to be rendered in an emacs buffer, which usually happens in the shell, emacs hangs and freezes, or takes too long to render the text compared to the terminal. What can be done to improve the performance of rendering text in emacs buffers?
As Drew says, your question is a bit vague. However, Emacs does have a long-known problem with long lines in files. Many of the internal operations that Emacs does make the assumption that lines are generally short. This is usually a safe assumption, of course.
As for exactly what is slow, the main problem is usually syntax highlighting. Most syntax highlighting is done with lots of repeated regular expression searches, which generally need to know the line boundaries to work. Turning off font-lock mode for that buffer frequently makes a huge difference.
I doubt the actual text rendering is a big part of the cost; simply rendering glyphs to the screen is generally pretty fast. Deciding which glyphs to render can take more time in some circumstances, but that's mostly related to the script the file is using. Anything that requires shaping will naturally be slower, for example. It still shouldn't be very significant.
If disabling font-lock mode doesn't help enough to satisfy you, I recommend profiling emacs with perf or similar tools. This will let you measure what's actually slow, rather than speculating. Then you may be able to make other adjustments to your configuration that will help.