Wrong! mass is extremely important. In the “real” world, as opposed to the imaginary world of theoretical physicists. It’s called support loss.
An amateur horologist (retired member of the supreme court bar) using a synchronome varied the mass W/O a changed shape, etc. by using Al, brass, and W bobs. both in its case and mounted on a load bearing wall.
As BrianWhatcott mentioned, there are a variety of devices considerably more accurate (greater resolution) than eyes and a stopwatch. If you wish ideas, I blow my horn:
My favo, a common theoretical problem done by the harassing prof. at Harvard and Cooper and Pellegrini “Modern analytic mechanics” pp. 116 ff. inter alii.
I never found it experimentally [1] so I did it: here:
p.s. Warning! No one has critiqued my work, except some of that published in the Horological Science Newsletter, where some “howlers' have been corrected.
Not incidentally This device [2] is used by the pro. horologist who attends to the, I’m certain, most famous world clock in the bell tower of the houses of parliament. (Big Ben refers to the largest bell.)
[1] I loath the expression “real world”.
[2] https://www.bmumford.com/mset/ <https://www.bmumford.com/mset/> I’ve advertised it several times on Phys-l and described its use to record pulses from a G-M counter to show the Poisson time distribution. An accurate measure of the dead time at a low counting rate is also a result. I think it’s also included in my web site.
Personal note. While instructing a pendulum lab at the community college in Ventura, CA, one student came to my desk puzzled, as had found his pendulum was not isochronous! “An A for you!” Don’t tell anyone; let’s see who else discovers it. etc.
Also Bryan M. attends to the clock in the County Court House Tower. He once was puzzled by the sudden change in the pendulum. Visiting he found the, probably, custodian had taken the quarters from the top of the pendulum!