Bash script, to do with grep, need the fastest solution possible

Hi everyone, need the fastest possible solution which involves
1) grepping through a lot of large files
2) using a list of strings to grep, a list rather large

So basically in details, I have several 46 very large files. Each of them contains the following lines
id:hash where
id is digital string, hash is sha1/md5I

need to process file with only ids and find id:hash pair for each of them in those large txt files.
Also I need to find exact pair. For example for 1234 simple grep would return 51234:, 5231234: etc

What's the fastest way to do it? The list with ids only contains about 30k rows, and the id:hash files each contain about 400k rows. I do use parallel which does increase performance, but since I need to find a very large set of ids, it's all very slow.

Any help would be greatly appreciated!

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Bash script, to do with grep, need the fastest solution possible