By 1944, the Soviet Air Force was very strong and very effective. This was because of improvements in the quality of the Soviet aircraft and better training and doctrine for the personnel.
At the start of the war, the Soviets had relatively poor airplanes and did not use what they had very well. Their pilots were poorly trained and the air force did not do a good job of protecting planes on the ground from German air attacks.
By 1944, however, this had changed completely. By that point, the USSR had two very effective fighters for use against the Germans. These were the Yak-3 and the LA-7. By the end of the war, these excellent airplanes were being produced in enough numbers that the USSR gained air superiority over the Eastern Front.
The USSR's Air Force, then, was very strong by the end of WWII. This strength came not from Lend-Lease, for the most part, but from the creation of newer and better airplanes within the USSR and from better training of its personnel.