This is probably the same as #218 and #368, but I felt that they might not describe the problem adequately.

Terminal output is too slow on many different occasions. When Termux cannot keep up with produced output, it stutters at best and becomes unresponsive for minutes at worst. As far as I can tell, using ANSI escapes (for colors, ncurses ui, etc.) makes things even worse.

Most typical manifestations in my experience:

• Scrolling in less and similar utilities. Scrolling down is fine, scronling up redraws whole screen and lags like hell.
• Build logs. Use minimal verbosity and you should be fine. Up verbosity a bit or get a lot of warnings due to a typo in a header, and your terminal is busted. Screen updates once every few seconds, and you'll be catching up with an already finished build for many minutes. (I usually just kill Termux and reconnect to my tmux session.)

This has been happening for a long while, half a year at very least. However, after one of the most recent updates (noticed on 0.60), the problem got much worse. On my Samsung tablet running Android 5.1.1, 80x25 terminal is mostly bearable, while on 190x40 vim takes almost a minute to start and regularly hangs for a few seconds (e.g. on newline entry). Not nearly as bad on my phone running 6.0.1, but still noticeable.

I haven't yet looked at source code (don't have time right now, neither do I have Android app development experience), but I intend to unless it's fixed in a week or two. I suspect excessive syncing among pty and UI threads and maybe inefficient scrollback handling.