Take too long to signup other people

I’m assuming you and the sheet admin failed to read the documentation? As well as the notes about the support forum is not the place for feature requests?

Documentation is here, in case you missed the links at the bottom of every admin screen in the plugin:
https://stephensherrardplugins.com/docs/pta-volunteer-sign-up-sheets-documentation/

Here is a copy and paste of the appropriate section discussing General Settings and the live search feature, which would do what you want:

Enable Sign-up form live search? — If you check this box, admin users can use the public side to sign-up other volunteers for tasks. Start typing in a first or last name on the sign-up form, and matching users/volunteers will show in a drop down list you can select to automatically fill the fields. When you submit the sign-up, that sign-up gets assigned to that user (if they have a user account on your site) instead of to your admin account.
Live Search Tables — Select which tables to search for the above live search field. You can select WordPress Users, or the Sign-Ups table, or both.

Once you fill in the info the first time for a volunteer, the sheet admin can use live search to find & select that volunteer (from the Sign-Ups table if you aren’t using User Accounts) and automatically fill out the rest of the fields for you. As long as you don’t turn on the option to automatically clear expired sign-ups, then all the sign-up info will be retained in the database and can be found with the live search feature.

OMG, I recalling reading about live search when I first reviewed the setup but I don’t think I fully understood what it did and obviously I didn’t make the connection even after my test user said the data entry was too slow. And yet, even with the RTFM you gave me an extremely helpful reply. Thank you, thank you!!! We are already testing with the live search setting in place and I very much hope they keep this plugin in place.

=)