my vTiger 6.5 user is slow sending emails and freeze the CRM -- 2

I'm using vTiger 6.5 and I'm having an issue with it being slow when sending emails and it seems to freeze the CRM. This only happens while using my crm user account, if I change user account it works normal. I do have an external SMTP server configured Sendgrid but I don't think this is the problem since other users, which exactly use the same SMTP configuration don't face the same problem. I also tried to use a different internet connection but it doesn't help. This problem occurred last few days, I've never had this problem before. I need assistance getting my vTiger 6.5 to send emails quickly and reliably, and with no freezing of the CRM. Thank you.