I run on an eight year old Thinkpad. I run my own config now, but used Spacemacs for quite some time. I've never had problems with speed. I didn't use it with Elixir though.
Perhaps try disabling your layers one at a time to see where the problem is.
I don't think it is Elixir, it is Helm most likely as someone already mentioned. I am using Vim and Emacs and I do notice significant difference in response to commands.
Helm is super helpful, so most likely I will disable it and see how it goes.
You could try looking at using ivy instead of helm. Not tried it myself but there was a thread on the emacs reddit last week [1] where people said it's much faster.
