I'm happy to read that you want to take a look into Seaside. It's surely worth a look
Squeak is currently one of the slowest Smalltalks in many areas, and there is quite some work going on to change that, but for the time being, a penalty of several hundred per cent in execution speed compared to the major commercial smalltalks is the norm for many areas.
Seaside has seen some major performance and memory usage improvements over the last few versions (starting with 2.7 iirc), so it's much better than it was two or three years ago now. But I'd guess that the overall performance of Squeak is part of what you're seeing.
... but I hope it won't be too long before we see VAST 8 beta