How to improve this awk code to reduce processing time - awk

Question

I have 400 tab-delimited text files with 6 million rows in each file. Below is the format of the files:
### input.txt
col1 col2 col3 col4 col5
ID1 str1 234 cond1 0
ID1 str2 567 cond1 0
ID1 str3 789 cond1 1
ID1 str4 123 cond1 1
### file1.txt
col1 col2 col3 col4 col5
ID2 str1 235 cond1 0
ID2 str2 567 cond2 3
ID2 str3 789 cond1 3
ID2 str4 123 cond2 0
### file2.txt
col1 col2 col3 col4 col5
ID3 str1 235 cond1 0
ID3 str2 567 cond2 4
ID3 str3 789 cond1 1
I am trying to add values in $1 from the rest of the file1..filen to $6 in input.txt file by using:
conditions:
1. columns $2 and $3 as key
2. If the key is found in files1...filen then if $5>=2 add the value from $1 to $6 in the input file.
Code:
awk -F "\t" -v OFS="\t" '!c {
c=$0"\tcol6";
next
}
NR==FNR {
a[$2$3]=$0 "\t";
next
}
{
if ($5>=2) {
a[$2$3]=a[$2$3] $1 ","
}
}
END {
print c;
for (i in a) {
print a[i]
}
}' input.txt file1..filen.txt
The output from the above code is as expected:
Output.txt
col1 col2 col3 col4 col5 col6
ID1 str2 567 cond1 0 ID2,ID3,
ID1 str4 123 cond1 1
ID1 str1 234 cond1 0
ID1 str3 789 cond1 1 ID2,
However, the problem is that the code is very slow as it has to iterate each key in input.txt through 400 files with 6 million rows in each file. This takes several hours to few days. Could someone suggest a better way to reduce the processing time in awk or using other scripts.
Any help would really save lot of time.

Answer 1

input.txt
Sam string POS Zyg QUAL
WSS 1 125 hom 4973.77
WSS 1 810 hom 3548.77
WSS 1 389 hom 62.74
WSS 1 689 hom 4.12
file1.txt
Sam string POS Zyg QUAL
AC0 1 478 hom 8.64
AC0 1 583 het 37.77
AC0 1 588 het 37.77
AC0 1 619 hom 92.03
file2.txt
Sam string POS zyg QUAL
AC1 1 619 hom 89.03
AC1 1 746 hom 17.86
AC1 1 810 het 2680.77
AC1 1 849 het 200.77
awk -F "\t" -v OFS="\t" '!c {
c=$0"\tcol6";
next
}
NR==FNR {
a[$2$3]=$0 "\t";
next
}
{
if ( ($5>=2) && (FNR > 1) ) {
if ( $2$3 in a ) {
a[$2$3]=a[$2$3] $1 ",";
} else {
print $0 > "Errors.txt";
}
}
}
END {
print c;
for (i in a) {
print a[i]
}
}' input.txt file*
For the above input files it prints the below output:
AC0,AC1,
WSS 1 389 hom 62.74
AC1,
WSS 1 810 hom 3548.77 AC1,
WSS 1 689 hom 4.12
WSS 1 1250 hom 4973.77
It still prints the values in $1 from file1 and file2