1/ Thunderbird is slow, especially when you scroll your emails. In CardBook preferences, first tab, check “Unique source for contacts”, if this is not enough, in Thunderbird preferences, Display tab, then Advanced tab, uncheck “Show only display name for people in my address book”.
2/ check in your console (CTRL+SHIFT+J), if you have indexedDB errors at Thunderbird startup, check this : https://cardbook.6660.eu/forums/topic/could-not-delete-address-or-edit-address-books-after-thunderbird-upgrade/
3/ contact edition is impossible, check this : https://cardbook.6660.eu/forums/topic/strange-bug-new-contacts-impossible/
4/ for others problems, check the CardBook log (for more log, increase the log size and enable the debug mode in CardBook preferences, first tab). To get the CardBook log, in the CardBook tab, click on the text at the bottom right. A common problem here is to have a return code of 0 for its web requests : this is due to a problem of network (proxy|firewall|router …)
5/ if nothing applies, send me an email to firstname.lastname@example.org
bests regards 😀