process-adaptive-read-buffering

process-adaptive-read-buffering is a variable defined in `C source code'.
Its value is

t

Documentation:
If non-nil, improve receive buffering by delaying after short reads.
On some systems, when Emacs reads the output from a subprocess, the output data
is read in very small blocks, potentially resulting in very poor performance.
This behavior can be remedied to some extent by setting this variable to a
non-nil value, as it will automatically delay reading from such processes, to
allow them to produce more output before Emacs tries to read it.
If the value is t, the delay is reset after each write to the process; any other
non-nil value means that the delay is not reset on write.
The variable takes effect when `start-process' is called.