I can confirm this bug. It still exists in 4:16.08.3-1. I found a solution for this here: https://bugs.kde.org/show_bug.cgi?id=346248#c1
According to this thread: https://bugs.kde.org/show_bug.cgi?id=350336 The bug also seems to be closed in the upstream version 16.12 but I haven't tested this yet. I've attached a debdiff containing the patch from the first thread. I've tested this for a while without problems.
diff -Nru ktouch-16.08.3/debian/changelog ktouch-16.08.3/debian/changelog --- ktouch-16.08.3/debian/changelog 2016-11-23 21:17:22.000000000 +0100 +++ ktouch-16.08.3/debian/changelog 2017-01-11 00:25:21.000000000 +0100 @@ -1,3 +1,10 @@ +ktouch (4:16.08.3-1.1) UNRELEASED; urgency=medium + + * Non-maintainer upload. + * Patched typing delay + + -- Nikolas Hemmert <nm...@web.de> Wed, 11 Jan 2017 00:25:21 +0100 + ktouch (4:16.08.3-1) unstable; urgency=medium * New upstream release (16.08.3) diff -Nru ktouch-16.08.3/debian/patches/series ktouch-16.08.3/debian/patches/series --- ktouch-16.08.3/debian/patches/series 1970-01-01 01:00:00.000000000 +0100 +++ ktouch-16.08.3/debian/patches/series 2017-01-11 00:09:09.000000000 +0100 @@ -0,0 +1 @@ +typing_delay_fix.patch diff -Nru ktouch-16.08.3/debian/patches/typing_delay_fix.patch ktouch-16.08.3/debian/patches/typing_delay_fix.patch --- ktouch-16.08.3/debian/patches/typing_delay_fix.patch 1970-01-01 01:00:00.000000000 +0100 +++ ktouch-16.08.3/debian/patches/typing_delay_fix.patch 2017-01-11 00:22:37.000000000 +0100 @@ -0,0 +1,34 @@ +Solves problem with big delay while typing. + +The solution found here: +https://bugs.kde.org/show_bug.cgi?id=346248#c1 + +--- a/src/declarativeitems/lessonpainter.cpp ++++ b/src/declarativeitems/lessonpainter.cpp +@@ -236,8 +236,7 @@ + const QString referenceLine = m_trainingLineCore->referenceLine(); + const QString actualLine = m_trainingLineCore->actualLine(); + const QString preeditString = m_trainingLineCore->preeditString(); +- const QTextBlock block = m_doc->findBlockByNumber(m_currentLine + 1); +- const int blockPosition = block.position(); ++ const int blockPosition = m_doc->findBlockByNumber(m_currentLine + 1).position(); + + for (int linePos = 0; linePos < referenceLine.length(); linePos++) + { +@@ -257,8 +256,14 @@ + + cursor.setPosition(charPosition, QTextCursor::MoveAnchor); + cursor.setPosition(charPosition + 1, QTextCursor::KeepAnchor); +- cursor.deleteChar(); +- cursor.insertText(QString(displayedChar), charFormat); ++ ++ /* QUICKFIX: Using the charFormat to decide whether a char must be repainted or not. ++ * Not nice but quite efficient ... */ ++ if (cursor.charFormat() != charFormat) ++ { ++ cursor.deleteChar(); ++ cursor.insertText(QString(displayedChar), charFormat); ++ } + } + + invalidateImageCache();
