Saurabh, thanks for running these tests.
The next step is for you to make a patch to remove the recommended tests from the EXCEPT_SLOW= list. It would also be useful to annotate the remaining EXCEPT_SLOW tests with their test times, as is done in recovery-small, so that the information you captured here is available in the future.
In terms of tests to enable, I'm generally in favour of your recommendations. For recovery-small test_51 it makes sense to enable this for ZFS as well, since it isn't much over 5 minutes, and it keeps the testing coverage consistent. It doesn't make sense to enable sanity test_51b without enabling test_51ba, since that would leave 70000 files in the filesystem and may cause later tests to run out of inodes.
In addition to figuring out which tests take a long time, it might be a worthwhile exercise to understand why the tests are taking so long (if that is clear from reading the test script, don't spend hours on it), and seeing if there is some way to actually reduce the test time rather than just skipping it.
For sanity 24D it might make sense to make that test run as part of test_51b (or immediately thereafter, e.g. test_51b0 similar) since test_51b has already created the required number of files and it would reduce runtime when SLOW=yes is selected. I agree test_24D takes too long to run for every test, so it shouldn't be enabled by default.
Also, have you looked at currently-running tests to see if any of them are taking too long and should be skipped (like LU-4537)?
reopening to correct fields to show up in 2.9 changelog