Solaris used to be quite slow, but that was many years ago. It's been reasonably fast since 2.5 and seems to have gained speed with each successive release.

Sun's compilers are quite good, but there's no way that I can think of to get a binary compiled for Solaris to run under Linux. Maybe there's some compatibility suite that I'm not aware of, though.

At the same time, Linux will be overwhelmingly faster on old Sparcs. The resources required to simply run Solaris are just too great for anything slower than, say, a Sparc5 (a 1994 or so era machine). That's because Sun's OS changed from what was commonly referred to as SunOS, which was a BSD derivative, to what's commonly called Solaris, which is an SVR4 derivative, and they seem to have kind of tuned their machines in those directions. (Which is really a load of hooey, but it feels that way.)