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

Job ID: 36327225

Budget: $10 – $30 USD

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 account, if I change 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 use the same SMTP don't face the same problem.
This problem occurred this 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.
Related categories: PHP Software Architecture CRM vTiger MySQL