How serious can this be? does it just effect performance

Welcome.

What engine? and is it a light on, a message or has someone read fault codes?

 

You will need to get the cars fault codes read to start with.

 

If you have just a Warning Message / EPC or Engine light on it can be as simple as spark plugs & misfires if a petrol, or a sensor, or just the petrol and maybe sitting ticking over too long as some point.

 

If a diesel it can be anything as well.

It is a 1 litre petrol engine, just have the symbol displayed in amber

Has your car had servicing recently, and has it had new spark plugs yet?

 

You need someone to plug in and read the fault codes if any. Someone that knows what they are looking at & not just clearing any codes if there are any.