Sky Broadband resells the Openreach fibre network with its own routers (Sky Hub / Sky Max). Performance is comparable to BT on the same line product; the differences are in router quality and customer service.
Sky measures speed at the router, not at your device. Run a wired test to confirm; if wired matches the plan, the issue is Wi-Fi coverage.
The older Sky Q Hub throttles when warm. Move it off shelves enclosed on three sides.
If your Sky Stream puck is wireless, it competes with everything else on 5 GHz. Hard-wire it via the included Ethernet adapter.