incorrect operation in multithreaded code

[Omeon]

Good day, please help me solve the following question.
I use zbar 0.23.92, Ubuntu 22.04 and C++11.
I am using a library for real-time barcode recognition. I am recognizing several images at once at the same time and for this I use several threads, in each of which I create a zbar::ImageScanner object.

But I noticed that this does not allow the scanner.scan(image) method to be performed at the same time. I took 2 images, noticed that in normal mode, each of them is recognized in 0.1 seconds. But when I create 2 streams and transmit one image to each, the first image is recognized in 0.1 seconds, and the second in 0.2 seconds. I think that during the recognition of the first image, the second stream cannot perform the scanner.scan(image) method.

Help in solving this issue.