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If you have 2Gbyte of free RAM, try
time awk -F: 'FILENAME=="f1"'NR==FNR { n[$0]++ ; next}; $1 in n ' f1file1 f2file2 > file3
time awk -F: 'FILENAME=="f1"{ n[$0]++ ; next} $1 in n ' f1 f2
awk -F: 'NR==FNR { n[$0]++ ; next}; $1 in n ' file1 file2 > file3
