The VGA driver has become unusably slow since r269471. I suspect that the support for non-8x8 fonts has dramatically increased the incidence of reads from VGA memory. It might be best if the driver could be refactored to avoid reads entirely, for example by passing some concept of write granularity up from the driver into the vt core so that vd_bitbltchr() is called only for operations of that width (redrawing partial characters).
Fixed by r270322
vt(4): Add new vd_bitblt_text_t callback, and implement it for vt_vga
Compared to the deprecated vd_bitbltchr_t callback, vd_bitblt_text_t
o the whole text buffer
o the dirty area
o the mouse cursor (map, position, colors)
This allows the backend to perform optimization on how to draw things.
The goal is to remove vd_bitbltchr_t and vd_putchar_t, once all driver
are converted (only vt_vga is included in this commit).
In vt_vga, this allows to draw the text and the cursor in one pass,
without ever reading from video memory (because it has all the context).
The main benefit is the speed improvement: no more slideshow during