Python search for array in large text file
Performance

It should be possible to accomplish this task in seconds rather than minutes, with the right data structure.  This is your main mistake:


paid = list(set(t))



The problem is, for a list with n items, it takes O(n) time to check whether the list contains a particular item.  It's particularly bad if the vast majority of the entries that you are seeking do not appear in the list — then it really does need to look at every single member of the list before concluding that an item is not in the list.

On the other hand, testing whether a particular element is a member of a set is O(1) — extremely efficient!

Miscellaneous

Please avoid cryptic filenames such as x and t.  Calling the file handle f is OK, since f is a typical name for a file handle, but since you have two files in this program, more distinctive names would help readability.  I would also avoid renaming the patent data file to f.txt, which is too cryptic.

It's a good habit to always call open() in the context of a with block, so that the file handle will be automatically closed for sure.  The 'r' mode is implied, and can be omitted.

Suggested solution

with open('data.txt') as paid_file:
    paid = set(line.rstrip() for line in paid_file)

with open('MaintFeeEvents_20160711.txt') as patent_events_file:
    for line in patent_events_file:
        if line.split(' ', 1)[0] in paid:
            print(line, end='')