You will find that entry level or lower spec leaf blowers do have reduced performance compared to a petrol counterpart, but don't let that put you off. The most important thing is matching your requirements to a suitable machine.
Professional grade leaf blowers are very powerful and provide matched performance to a petrol alternative. However, do consider that the cordless option is typically more expensive when factoring in the charger and battery options.
The main concern with cordless leaf blowers are the run time and battery charging time. However, as long as you choose your blower based on the work you need to do, cordless is a fantastic, clean alternative to a petrol engine.