Thunderbird became slowly, to update the mails needs a lot of time and in this time, I can not read anymore the other mails. What can I do? Thanks.

I have more google accounts and an yahoo one. To update the mails, thunderbird blocks everything and I lasts a lot. Sometimes it crashes, I sent the informations, but nothing till now, from none. I have Kaspersky antivirus, and Windows 8.1. Thanks a lot. Mihaela