Raza - 7:15 pm on Jun 2, 2010 (gmt 0)
I would guess the Express limitation that would be of most concern is the limit of one CPU (others are 1GB RAM for the pool and 4GB for the database size). If that CPU is shared by other processes (like IIS) and gets taxed by that other process, I can envision poor performance.
Even so, 700 visitors a day is pretty low volume. One of the unfortunate limitations of Express is that it doesn't come with Profiler, although you can run traces and import/analyze them. Normally, all things being equal, that's what I'd do to try to isolate the problem.