Notepad++ starts to lag with very long lines

Description of the Issue

When current screen with a text contains 8-12 lines with a length of each line more than 64K the application start to lag on navigation, the cursor becomes slow.

Steps to Reproduce the Issue

1. Create the new text file, fill it with text lines of standard lengths (80-120 character), in the amount of 100-200 lines for free navigation.
2. Now, create long lines with length more than 64K, it can be easily created by selecting a whole line and pressing multiple times Ctrl+D, until required length is achieved. Remove the selection and duplicate the long line ten times using the same hotkey Ctrl+D.

Expected Behavior

Navigation using cursor should be very fast as usually.

Actual Behavior

Navigation using cursor becomes very slow.

Debug Information

Notepad++ v7.5.1 (64-bit)
Build time : Aug 29 2017 - 02:38:44
Path : C:\Program Files\Notepad++\notepad++.exe
Admin mode : OFF
Local Conf mode : OFF
OS : Windows 7 (64-bit)
Plugins : AutoSave.dll mimeTools.dll NppConverter.dll nppcrypt.dll PluginManager.dll

Start by reproducing the lag with the described file containing 8–12 lines longer than 64K, using Ctrl+D to create and duplicate them. Profile cursor navigation while comparing normal-length and very long lines, then verify that navigation remains responsive in the reported scenario without regressing ordinary text movement.