I haven't figured it out exactly, but I understand that without any unrolling, it takes something like 66 clock cycles per hash, so at 50 MHz this is a little less that 1 Mhash/s. This would give about 1.5 h between shares in a pool, which is roughly what I see.

However, this is only about 60 % utilization, it's frustratingly close to being able to double this. (It would need about 10K vs. my 9K LUTs.) The next best Spartan3 has 1200K gates vs this 500K, so it might be able to quadruple the units.

I think you need a Spartan6 to do any serious mining, but even then you should check the number of logic units, the series has some low-end models as well.

Of course, you can also increase the clock frequency. But, for example, the current code on this chip is limited to about 70 MHz.