This has been my experience with rbenv, asdf and all other solutions that relies on shims.
Depending on what you are trying to do (for example running short lived CLI), you will incur into a slightly performance cost because of the bash/zsh/fish shell you need to spawn before running your code. In ruby's case rbenv/asdf adds around 20-50ms (could be more etc depending on your shell and your shells initialization code etc).