编了一个在安卓手机，终端上运行的Shell脚本，执行效率不高，运行耗时。想优化提速。
请高手指点。

# 查找小于300k且15天内没有被访问/修改过的文件(atim -n，n天内没被访问过;atim +n，n天之前没被访问过)

echo "   开始清理SD卡中影音图片小文件，稍等..."
smallfile_1[0]=`$find_2 $sd \( -path $sd/Books -o -path $sd/DCIM -o -path $sd/Download -o -path $sd/Pictures -o -path $sd/WechatXposed \) -prune -o \( -atime +15 -o -mtime +15 \) -type f -size -300k 2>/dev/null|sed -e '/.nomedia/ d' -e '/\/0\/Books/ d' -e '/\/0\/DCIM/ d' -e '/\/0\/Download/ d' -e '/\/0\/Pictures/ d' -e '/\/0\/WechatXposed/ d' -e '/\/0\/com.tencent.tmgp.sgame/ d' -e '/\/MIUI\/theme\/.data/ d'` &
smallfile_1[1]=`$find_2 /data/data \( -atime +15 -o -mtime +15 \) -type f -size -300k 2>/dev/null|sed -e '/.nomedia/ d'` &
for smallfile_2 in ${smallfile_1[*]};do
    { smallfile_3=`file "$smallfile_2" 2>/dev/null | $awk_2 '{print $2}'`;
    [[ $smallfile_3 == "empty" || $smallfile_3 == "BMP" || $smallfile_3 == "GIF" || $smallfile_3 == "ICO" || $smallfile_3 == "JPEG" || $smallfile_3 == "JPG" || $smallfile_3 == "PCX" || $smallfile_3 == "PNG" || $smallfile_3 == "SWF" || $smallfile_3 == "SVG" || $smallfile_3 == "TIF" || $smallfile_3 == "TIFF" || $smallfile_3 == "WEBP" || $smallfile_3 == "WMF" || $smallfile_3 == "3GP" || $smallfile_3 == "AAC" || $smallfile_3 == "ASF" || $smallfile_3 == "ASX" || $smallfile_3 == "AVI" || $smallfile_3 == "FLAC" || $smallfile_3 == "FLV" || $smallfile_3 == "MKV" || $smallfile_3 == "MPEG" || $smallfile_3 == "MP3" || $smallfile_3 == "MP4" || $smallfile_3 == "WMV" ]] && (rm -f \"$smallfile_2\" 1>/dev/null 2>>$sd/adzw_7_err.txt;[[ $? == "0" ]] && echo $smallfile_2 >>$sd/adzw_7.txt);} &
done