I haven't been able to track this to a particular component (e.g. a
regexp) for now, but font-lock-fontify-region is now considerably slower
than it was in Emacs 26 (at least at revision cb8fb597e5bf4f14).

To reproduce: install elixir-mode (e.g. from MELPA Stable):

(add-to-list 'package-archives
              '("melpa-stable" . "https://stable.melpa.org/packages/") t)

M-x list-packages, install elixir-mode.

Savet the attached tiny.__ex__ as tiny.ex.

Visit tiny.ex.

Eval: (benchmark 1 '(font-lock-fontify-region (point-min) (point-max))).

"Elapsed time: 0.158824s"

With larger files, the times are much longer.

I had a break from Elixir, so I noticed this only now.

In GNU Emacs 27.0.50 (build 11, x86_64-pc-linux-gnu, GTK+ Version 3.24.8)
  of 2019-11-05 built on potemkin
Repository revision: dd19cc3aa16ccc441a8a2bfcdeb3005a6eef2543
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Ubuntu 19.04