I have the same problem, though it doesn't last for 3-5 minutes, thankfully. According to my MyPhone page, I have 1174 messages. This doesn't sound like very many to me.
I notice in Task Manager that SMS takes over 2 MB. Maybe this is part of the reason for the sluggishness?
Another (minor) annoyance is that, in the Windows SMS app (I don't use Touch Flow), when I tap "reply" to a text, the program will take time to thread the messages from that sender and then load the SIP (I use Swype currently). Next, the SIP goes away and I'm left with the cursor at the bottom of the screen in a blank area for my reply. I have to reactivate the SIP and wait for the screen to redraw again before I can begin typing. Seems like weird behavior.