bug#25148:

[Eli Zaretskii <eliz@gnu.org>]

> From: skrechy@gmail.com
> Date: Wed, 14 Dec 2016 18:29:26 +0200
> 
> This is the output of of C-u C-x =:
> 
>              position: 2673 of 4393 (61%), column: 15
>             character: h (displayed as h) (codepoint 104, #o150, #x68)
>     preferred charset: ascii (ASCII (ISO646 IRV))
> code point in charset: 0x68
>                script: latin
>                syntax: w 	which means: word
>              category: .:Base, L:Left-to-right (strong), a:ASCII, l:Latin, r:Roman
>              to input: type "C-x 8 RET HEX-CODEPOINT" or "C-x 8 RET NAME"
>           buffer code: #x68
>             file code: #x68 (encoded by coding system utf-8-unix)
>               display: by this font (glyph code)
>     uniscribe:-outline-Courier New-normal-normal-normal-mono-13-*-*-*-c-*-iso8859-1 (#x4B)

Are you saying that scrolling is slow even if the buffer contains only
ASCII characters, such as 'h'?  Are there no non-ASCII characters in
that buffer?  If there are non-ASCII characters, can you tell which
font(s) is/are used for them?