In Solaris 2.4 my TCP performance is extremely poor.

Patch 101969-05 broke TCP/IP throughput. You need to backout this
patch or obsolete it with the kernel jumbo patch (101945-27 or
later). The latter is recommended.

On Solaris 2.4 x86, later kernel jumbo patches (101946-29) triggered
bugs in some of the ethernet cards. This manifests itself as extremely
poor TCP throughput. On such systems, you need to install DU10 or
later.