Yes, a mesh router will need to use a significant chunk of its overall capacity to link the mesh nodes. This usually works out to around half of your 5GHz Wi-Fi speed. Getting a system with a dedicate backhaul will offer the most consistent speeds since it will have an entire 5GHz band just for your devices.
If you're picking out a dual-band system, make sure the 5GHz bands can deliver at least double the internet speed your want to see on your devices. Tri-band systems are a bit more complicated but they have a few more options when it comes to configuration. Make sure the band that's dedicated to devices has plenty of speed for your needs. If it doesn't use a dedicated band, like the TP-Link Deco X90,  stick to the same rule as dual-band systems and make sure you have around twice the speed you need on your devices.
If you have an internet connection under 500Mbps or so, an AX1800 kit with a 1201Mbps 5GHz band should have most of the speed you need but still may struggle to achieve 500Mbps in congested areas. This is one of the reasons we have chosen a faster AX3000 mesh kit as out top pick.