The brief story is that I was investigating my startup performance and aside from my vimrc, joshdick/onedark.vim along with polyglot turned out to be the biggest offender. Since compatibility with vim is not that important for me, there is no good reason to spend so much time on initializing a color scheme and I just rewrote the entire thing in lua to see if it helps. And it did helped, it's now about 3 times faster.

If it wasn't obvious from the plugin name and the mention of lua, it requires neovim and probably at least version 0.5.0. Options g:onedark_color_overrides, g:onedark_termcolors, g:onedark_hide_endofbuffer and g:onedark_terminal_italics are all ported. I didn't bother to port functions, like onedark#extend_highlight etc. Works with lightline, but I haven't tested it with airline.

if has('nvim-0.5.0')
  Plug 'ii14/onedark.nvim'
else
  Plug 'joshdick/onedark.vim'
endif

All the credits goes to the author of the original plugin, joshdick/onedark.vim.