What is the easiest way to decrease the linespread in a table of contents?

You could use the tocstyle package that comes with the KOMA Script bundle. But you will get a warning about the Alpha state of the package so you should use it with caution. At least the below code works as desired.

\documentclass[english]{scrartcl}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{tocstyle}
\usepackage{babel}
\usepackage{blindtext}

\setcounter{tocdepth}{1}
\newtocstyle{compact}{%
  \settocfeature[1]{entryhook}{\bfseries}%
  \settocfeature[1]{entryvskip}{0pt plus 2pt}%
  \settocfeature[1]{leaders}{\hfill}%
}
\usetocstyle{compact}

\begin{document}
  \tableofcontents

  \bigskip
  \Blinddocument
\end{document}

And as always, the blindtext package is only for creating dummy text thus not part of the solution.

---

You may use the etoolbox package to patch the command \l@section which is (beside other things) responsible for setting the vertical space before a section entry in the ToC.

\documentclass{scrartcl}

\usepackage{etoolbox}
\makeatletter
\patchcmd{\l@section}{1.0em}{\z@}{}{}
\makeatother

\setcounter{tocdepth}{1}

\begin{document}

\tableofcontents

\section{bla}

\section{blubb}

\section{foo}

\subsection{bar}

\end{document}

Since KOMA-Script 3.20 you can use option tocbeforeskip of command \RedeclareSectionCommand to change the distance before a TOC entry:

\documentclass{scrartcl}[2016/05/10]% Needs at least KOMA-Script 3.20!
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{blindtext}

\setcounter{tocdepth}{1}
\RedeclareSectionCommand[tocbeforeskip=0pt]{section}

\begin{document}
\tableofcontents

\blinddocument\blinddocument\blinddocument\blinddocument
\blinddocument\blinddocument\blinddocument\blinddocument
\end{document}