cats decrease performance and make petrol consumption rise
Correct.
The reason is simple too, here's why.
Any engine pumps gas, it is basically an airpump.
An intake filter is necessary, but luckily they can & do make them big enough so that resistance, hence performance, is not really hampered.
An exhaust catalyst (or particulate filter) is a HUUGE RESISTANCE for the gasflow, and the engine must provide the energy to pump through it - so
it uses additional fuel for doing
only this!
They cannot make them bigger because then the temperature will become too low, they have to keep them smallish.
Remove the Cat/EPF - and witness more performance, less fuel consumption and a load less heat.