Message sending to internet is slow

I have configured a send connector to my exchange 2010 after following MS KB.  but the problem is it is very slow responding. I checked in queue after message sending, it takes more than a minute to go to internet from my exchange server. Is there any way to reduce that time. Now it is taking around 5 mins to send to internet. Below is my send connector settings:

[PS] C:\Windows\system32>Get-SendConnector -Identity external | fl

AddressSpaces                : {SMTP:*;1}
AuthenticationCredential     :
Comment                      :
ConnectedDomains             : {}
ConnectionInactivityTimeOut  : 00:05:00
DNSRoutingEnabled            : True
DomainSecureEnabled          : False
Enabled                      : True
ErrorPolicies                : Default
ForceHELO                    : False
Fqdn                         : webmail.externaldomain.com
HomeMTA                      : Microsoft MTA
HomeMtaServerId              : HUBCASSERVER1
Identity                     : External
IgnoreSTARTTLS               : False
IsScopedConnector            : False
IsSmtpConnector              : True
LinkedReceiveConnector       :
MaxMessageSize               : 10 MB (10,485,760 bytes)
Name                         : External
Port                         : 25
ProtocolLoggingLevel         : None
RequireOorg                  : False
RequireTLS                   : False
SmartHostAuthMechanism       : None
SmartHosts                   : {}
SmartHostsString             :
SmtpMaxMessagesPerConnection : 20
SourceIPAddress              : 0.0.0.0
SourceRoutingGroup           : Exchange Routing Group (DWBGZMFD01QNBJR)
SourceTransportServers       : {HUBCASSERVER1, HUBCASSERVER1}
TlsAuthLevel                 :
TlsDomain                    :
UseExternalDNSServersEnabled : True