Best practice for plugins which are not hosted on wordpress.org is to hook into the wordpress update workflow, and do the version check request at that time. This means that updates are being checked only when core decides that it is time to check, which usually happens at an admin page load once every 7 days.
License related checks of plugins that follow the best practice should not add any measurable time to front end page load time. If you have many such plugins, they might from time to time slow the load of an admin page, but it probably depends more on the quality of your server, and the plugin’s server, how fast can the DNS be resolved and the communication done.
How likely it is to be noticeable? I read only once about someone that had complained about it.
This was in general, as for your specific setting, if you worry about it you should contact the relevant authors and ask them.