Timing word-searching code in Python
Some suggestions:


Pass the code through pycodestyle to make it more idiomatic.
Use with open() (or try/finally for a more complicated solution) to make sure files are closed whenever they are done with.
You can do .readlines() on a file pointer to avoid stripping newlines.
The dictionary is presumably controlled by you. Sort and lower-case it once, and then you don't have to do that every time the script runs.
These are logically equivalent:

    for x in y:
        if x == target:
            return True
    return False


and

    return target in y

Use descriptive names such as for word in words and `def linear_search.
Put the timing code around the lines you are trying to time. If you want to know the aggregate time of linS then get the start and end times just before and just after the last loop (which will count the time it takes to write to disk buffers) or accumulate a sum of run times within `linS`.