How to improve the looks of a table?

Don't use vertical lines at all:

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath, amsmath, amssymb}
\usepackage{caption}
\usepackage{ctable}
\usepackage{multirow}
\begin{document}

\begin{table}[H]\centering
\caption{\label{tab2} FMM: profiling of subroutines}
    \begin{tabular}{lcccccc}\FL
    \multirow{2}{*}{$\Theta$} & \multicolumn{6}{c}{N} \NN
    \cmidrule(rl){2-7}
         & 100    & 200    & 400    & 800    & 1600   & 3200   \ML
     0.1 & 0.0008 & 0.0020 & 0.0056 & 0.0180 & 0.0556 & 0.1708 \\
     0.2 & 0.0008 & 0.0016 & 0.0036 & 0.0108 & 0.0300 & 0.0792 \\
     0.4 & 0.0004 & 0.0012 & 0.0028 & 0.0060 & 0.0140 & 0.0344 \\
     0.8 & 0.0004 & 0.0008 & 0.0016 & 0.0036 & 0.0064 & 0.0148 \\
     1.6 & 0.0004 & 0.0004 & 0.0008 & 0.0028 & 0.0036 & 0.0080 
    \LL
    \end{tabular}
\end{table}
\end{document}

no vertical lines, no centering, friendlier spacing:

\documentclass[12pt]{article}
\usepackage{booktabs,leading}
\usepackage[utf8]{inputenc}
\usepackage[justification=raggedright,singlelinecheck=false]{caption}

\begin{document}

\begin{table}
\leading{16pt}
\caption{\label{tab2} FMM: profiling of subroutines}

    \begin{tabular}{llllllll}
    \cmidrule[0.08em](r){1-2}\cmidrule[0.08em]{3-8}
    & & \textbf{N} \\
     &  & \textbf{100} & \textbf{200} & \textbf{400} & \textbf{800} & \textbf{1600} & \textbf{3200} \\
    \cmidrule[0.08em](r){1-2}\cmidrule[0.08em]{3-8}
    $\Theta$ & 0.1 & 0.0008 & 0.0020 & 0.0056 & 0.0180 & 0.0556 & 0.1708 \\
    & 0.2 & 0.0008 & 0.0016 & 0.0036 & 0.0108 & 0.0300 & 0.0792 \\
    & 0.4 & 0.0004 & 0.0012 & 0.0028 & 0.0060 & 0.0140 & 0.0344 \\
    & 0.8 & 0.0004 & 0.0008 & 0.0016 & 0.0036 & 0.0064 & 0.0148 \\
    & 1.6 & 0.0004 & 0.0004 & 0.0008 & 0.0028 & 0.0036 & 0.0080 \\
        \cmidrule[0.05em](r){1-2}\cmidrule[0.05em]{3-8}
    \end{tabular}
\end{table}
\end{document}