It seems that I cannot reference tables when they are placed inside onlyenv, but . This applies to both of \cref and the traditional \ref
The topic Table numbers in beamer is not an answer to this question because my question is asking about referencing a table, not inserting its number in a caption.
Here is the MWE. This should be run twice to get the real result.
\documentclass[aspectratio=169, xcolor={x11names}]{beamer}
\usecolortheme{seahorse}
\useoutertheme{miniframes}
\useinnertheme{inmargin}
\usefonttheme[onlylarge]{structurebold}
\setbeamertemplate{caption}[numbered]
% Command to change margins for a page wide
\makeatletter
\newcommand{\framePageWide}{
\def\Gm@lmargin{5mm}%
\def\Gm@rmargin{5mm}%
\textwidth=\dimexpr\paperwidth-\Gm@lmargin-\Gm@rmargin\relax
\hsize\textwidth
\columnwidth\textwidth
\setlength{\linewidth}{\textwidth}
\hoffset=\dimexpr-\beamer@leftsidebar+\Gm@lmargin-0mm\relax
}
\makeatother
\newcommand{\framePageWideEnumI}{\setlength{\leftmargini}{10mm}}
\usepackage{adjustbox}
\usepackage{ragged2e}
\usepackage{tabularx}
% BEGIN_FOLD
\newcolumntype{C}{ >{\arraybackslash \Centering}X }
\newcommand{\scalelinespace}[1]{
\rule{0pt}{#1\normalbaselineskip}
}
% END_FOLD
\usepackage{booktabs}
\usepackage{makecell}
\usepackage{amsmath}
\usepackage{amssymb, amsfonts}
\usepackage{cleveref}
\crefformat{table}{Table #1}
\begin{document}
{\framePageWide
\begin{frame}{Tables}
\begin{onlyenv}<1>
\framePageWideEnumI
\begin{itemize}
\item Item 1
\item Item 2
\end{itemize}
\end{onlyenv}
\begin{onlyenv}<2>
\begin{table}
\begin{adjustbox}{minipage={0.5\textwidth}, valign=t}
\caption{Time, in seconds, needed to calculate a single ${Z}_{bus}$ Element}
\label{table:results2}
\begin{tabularx}{\linewidth}{c C C}
\toprule
System Size & Our Method & Brown's Method
\\
\addlinespace
200-bus & $4.3\times10^{-6}$ & $0.092$
\\
\Xhline{0.25mm}
2383-bus & $3.8\times10^{-6}$ & $166.2$
\\
\Xhline{0.25mm}
3120-bus & $3.2\times10^{-6}$ & $398.8$
\\
\bottomrule
\end{tabularx}
\end{adjustbox}
\hfill
\begin{adjustbox}{minipage={0.43\textwidth}, valign=t}
From \cref{table:results2} we see that
\begin{itemize}
\item Our method is fast
\item By contrast, Brown's becomes very slow when the system is large
\end{itemize}
\end{adjustbox}
\end{table}
\end{onlyenv}
\end{frame}
}
\end{document}

\label<2>{...}. – Salim Bou Mar 25 '19 at 06:30