I have built emacs 27 from master yesterday (2019-08-30). According to
`ps u`, it cosumes at least 3 times more memory than emacs 26.3, soon
after it's started, and regularly becomes irresponsive, several hours
after start. Looks like it is always triggered by a user action, even if
insignificant, i.e. emacs doesn't seem to freeze when idle. When it
does, it makes the whole system significantly less responsive, and has
to be killed.

Compiled with harfbuzz 2.3.1 and imagemagick 7.0.8.56, without cairo. I
could try different combinations of configure options and report further
if needed.

I run two emacs instances, a GUI instance as window manager (exwm), and
a daemon for emergency cases. Both instances consume more memory than
v26 counterparts did; however, GUI instance gets there sooner than the
other one.

Otherwise, 27 feels more stable than 26 and faster too, especially
w.r.t. startup time in tty and completion responsiveness (helm) in GUI.