The answer is not straightforward.
The Hub 4 can run at up to 2.5 Gbps (VM did a limited trial a while back at this speed as I recall), and in normal use on a 1 Gbps connection can achieve 1170 Mbps. But that's always constrained at a device level because the cheapskates who specified the Hub 4 equipped it with 1 Gbps ports and 802.11ac wireless. What that means is that in router mode, the Hub 4 can achieve up to 1.17 Gbps, spread across more than one device in any combination that you like, but can only achieve a peak device speed of 1 Gbps for any ethernet device, and around 600 Mbps for any wireless device (heavily dependent upon the device itself, some go higher, most are lower).
If you run the Hub 4 in modem mode, then your router only gets the 1 Gbps from a single ethernet port, so (potentially) you're starting off 15% down on the total speed that the Hub 4 in router mode could achieve from aggregated devices. Your "per device" limits will then depend on the LAN port speeds, and the wifi protocol it uses. A decent third party router will often outperform the pound shop router built in to the Hub 4, but there's been a modest number of odd problems reported in the forum where Hub 4's struggle to achieve good output speeds to a third party router.
So which is faster? It all depends. Choose what works for you - as a general rule I'd expect using a third party router would be better because the Hub 4's wifi is unexceptional.
