Well, I am also running 2 terminals and a server with SQL Express, no speed issues on the terminals at all. The server does however show some strain, but that is because of SQL Server running of it, not because of SambaPOS, and of course it is not a state of the art pc, little bit old, sorry ..... very old, it is only a Celeron 2.4GHz with 1GB RAM and I push it too the limit with all the other stuff I do as well.
Point is that SambaPOS does not cause any issues at all.