一般的来说前面的that are需要省略的,因为n+that be+adj的形式是不简洁的。所以that are省略,后面的are补出来的原因则是因为时态不同的原因吧,前面句子的逻辑意思是说:an alliance would enable customers to receive…at speeds much higher than are currently possible.我觉得前面是would,可能性,也就是说前面的逻辑意思应该是at higher speeds that would be much higher than (that )are currently possible. Are是代表current现在的意思,所以时态不同,需要补出动词的,而比较的主语对象是speeds一样的,所以这里省略主语,但是动词不省略。