Well, even with the best circumstances, you're never likely to see 200Mb/s, let alone 450Mb/s. There are a lot of factors that explain why you'll never get close to theoretical advertised speeds. Overhead, interference, distance, duplex, access point performance, band, etc. Or yeah, the tablet could always be the bottleneck.
I assume you have an AC router? But are you also on 5GHz or 2.4? That can make a huge difference. How far from the AP are you? Have you tried changing channels?