It is true that some ETFs have larger spread than is typical on very liquid stocks. I sometimes use hl.co.uk to research the spread before I buy. For example, they say that for VERX “Indicative spread: 0.22%”. Taking a less liquid one, SUWS, Indicative spread: 0.43%. At the better end, ISF and VUSA both have Indicative spread: 0.07%, which makes them popular with day traders.