Does it actually browse very slowly and or sometimes time out, or does it just not work when you add the HTTP profile? There could be several things amiss here:
1. If you need to re-encrypt traffic to the back end server and you don't have a server SSL profile applied, it's more than likely not going to work at all.
2. If you have a 443 virtual server pooling to 443 servers, with no SSL profiles (SSL pass through), then you cannot add an HTTP profile. In fact if you're not terminating the client side SSL then you cannot use an HTTP profile.
3. You said "the standard http profile we have". Does that mean you've modified the default HTTP profile? If so, what did you change?
4. It could be that the application uses absolute addressing and cannot handle the https:// namespace when it's listening on http://. You need to look at what the server is sending to the client. If it's sending references to http:// resources (redirects, page objects, etc.), which the client cannot access, then you'll need to make provisions for that.