OMZ is fast even on my phone.

It turned slow for a whole after I installed anaconda. If you use conda and your shell is slow, try commenting out the code that loads conda, because it's probably that.

nvm likes to slow things down too. Not as bad as conda, but I did add some code to delay load it.

Do you know how fish would prevent getting slowed down by external scripts in its configuration file? Or does it just not support those scripts perhaps?