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Syscom the P38 wasn't 100 mph faster than anything in the air.
Marcel the Me-262's were seriusly outnumbered, mostly piloted by inexperienced pilots, and the few which were available couldn't always fly because of fuel shortages. That's the reason it had little impact on the outcome of the war.
However the headline of the topic is "Which fighter gave the best new advantage when introduced?" which would be the Me-262 without any doubt as it was the biggest leap forward in a/c technology of the entire war. One on one and with an experienced pilot the Me-262 was unbeatable.
No one said it was the most effective, but it was had a big advantage with speed. And as Erich pointed out, bomber crews were very concerned with the Me-262.
Does anyone know if there was another bird out there, in service, using swept wings?
I-16 - first all - metal fighter monoplane with the retractable landing gear
Me 163.