Costco management does use splinter notes to document you received training in the methods that they created. Splinter notes should not be used to document whether or not an employee is working fast enough.
A sense of safety always overrides a sense of urgency. Management can’t discipline you for working according to their rules and methods. The age and physical condition of an employee must be taken into consideration. If you feel they are not, then you have the right to file a claim with Equal Employment Opportunity Commission (EEOC).
Costco management can and will direct their employees to sign splinter notes. Splinter notes are not covered in the Costco contract so you are not required to sign. If you are directed to sign splinter notes, please remember to write that you are signing under threat of discipline on the document. Make sure you get a copy of all splinter notes which can be used to document their harassment.