Speed up the conversion of MV-HEVC to Side-by-side

I have read the Converting side-by-side 3D video to multi-view HEVC and spatial video, now I want to convert back to side-by-side 3D video. On iPhone 15 Pro MAX, the converting time is about 1:1 as the original video length.

I do almost the same as the article mentioned above, the only difference is I get the frames from Spatial video, merging into Side-by-side. Currently my code merging the frame wrote as below. Is any suggestion to speed up the process? Or in the official article, is there anything that we can do to speed up the conversion?

            // Merge frame
            let leftCI = resizeCVPixelBufferFill(bufferLeft, targetSize: targetSize)
            let rightCI = resizeCVPixelBufferFill(bufferRight, targetSize: targetSize)
            let lbuffer = convertCIImageToCVPixelBuffer(leftCI!)!
            let rbuffer = convertCIImageToCVPixelBuffer(rightCI!)!
            pixelBuffer = mergeFrames(lbuffer, rbuffer)