cooling system comprise of many supporting components. the pressure build-up in such system must be properly maintained in order for it to be efficient.
if water level never reduced before, at least there's good news. so, check for other things
as u mentioned temperature increase only when going high speed.
i suspect
1. radiator fan motor not powerful enough so, fan is turning slower than it should be.
2. thermostat or radiator cap jammed thus not allowing expansion internally or externally.
3. but the real favourites are clogged radiator tubes. its hardly seen from outside. cloggs are made up of rust and one good indicator is by checking radiator internals (when its cool lah..)
if its rusty and the tubes look like sumbat already, that must be the culprit then. especially if u never use properly mixed coolant.
air, water and metal cant mixed. otherwise, the answer is rust. and rust is never a good friend of car cooling system.
good luck!
