Please consider purchasing our new software.
　 　　It is so fast and therefore is save time.

　　It is so fast and therefore saves time.

　　　　請考慮我們的新軟體。它非常快速因此很省時。

And為對等連接詞，用來連接 is 和 saves 兩個動詞（主詞為 it ）；
is save 為錯誤動詞結構。