Can I use Titan XP on MacBook?
No you really need a Thunderbolt connection.

It wasn't until Thunderbolt 3 that the bandwidth of the protocol fully caught up with the idea. Realizing this, the group responsible for the spec created implementations with the possibility of some configurations being able to pass the video signal back to the computer it was boosting.

Using a External GPU with older Thunderbolt connections will not fully utilise the performance increase of an External GPU.