So you already have the Polygon Siskiu T8 and are looking to add the Anthem to the stable? Or replace the Siskiu?
Sure, the Anthem is going to be a faster bike to pedal than the Siskiu T8. But a lot of that will simply be the difference in tires. DHR is definitely not a fast rolling rear tire, and DHF is only slightly better. You can always try faster rolling tires (especially in the rear). And in 2.6, those are pretty heavy as well. You could look at tires in the 2.4ish range. Just making a smart tires swap can get you a faster bike if that is what you are looking for.
But even besides the tires, the Anthem is a better bike to get from point A to B the fastest if there is pedaling involved (thus it is an XC race bike). Whether you prefer that kind of bike is another story, though. Really depends what you are riding, and what you like to focus on.
The Siskiu T8 hits that mid-travel sweet spot that is generally a real crown pleaser. Its what I ride (135/150). Yeah, I would have better Strava times in the climbs and flats with an XC bike, but its not as fun for me pointing downhill.