Air traffic at National Airport returned to normal yesterday afternoon after air traffic controllers apparently ended a slowdown which had caused major delays and cancellations on Tuesday and Wednesday, Federal Aviation Administration officials said.

FAA spokesman Fred Farrar said controllers continued the slowdown until about 2 p.m. Traffic to the Northeast remained behind schedule after that because of weather conditions in New York and Boston, Farrar said.

A spokesman for Eastern Airlines, which had been forced to cancel its New York shuttle Wednesday evening reported delays of up to 45 minutes last night. The spokesman said the delays were normal for the weather conditions.

Other airlines reported similar delays to New York but said flights were on time to other parts of the country. On Wednesday they had reported delays up to two hours.

FAA officials, who had avoided the word "slowdown" on Wednesday, said yesterday that the controllers had been delaying flights.

The Professional Air Traffic Controllers Organization has denied the slowdown had taken place but warned of "frustration and bitterness on the part of many controllers" resulting from a dispute over "familiarization flights" overseas.

The latest FAA contract with PATCO provides for free flights abroad in the cockpit so that controllers may observe in-flight techniques. PATCO has complained that it has had no official response from Pan AM and Trans World Airlines its familiarization program. The airlines responded that they have no objection to the flights but cannot guarantee a cockpit seat for a controller.