一、问题

GPUImage第一次渲染的时候卡顿。
日志：Main Thread Checker: UI API called on a background thread: -[UIView bounds]
原因：在子线程访问UI的属性。
二、解决
返回主线程。

- (void)recalculateViewGeometry;
{
    dispatch_async(dispatch_get_main_queue(), ^{
        
        runSynchronouslyOnVideoProcessingQueue(^{
            CGFloat heightScaling, widthScaling;
            
            CGSize currentViewSize = self.bounds.size;
            
            //    CGFloat imageAspectRatio = inputImageSize.width / inputImageSize.height;
            //    CGFloat viewAspectRatio = currentViewSize.width / currentViewSize.height;
            
            CGRect insetRect = AVMakeRectWithAspectRatioInsideRect(inputImageSize, self.bounds);
            
            switch(_fillMode)
            {
                case kGPUImageFillModeStretch:
                {
                    widthScaling = 1.0;
                    heightScaling = 1.0;
                }; break;
                case kGPUImageFillModePreserveAspectRatio:
                {
                    widthScaling = insetRect.size.width / currentViewSize.width;
                    heightScaling = insetRect.size.height / currentViewSize.height;
                }; break;
                case kGPUImageFillModePreserveAspectRatioAndFill:
                {
                    //            CGFloat widthHolder = insetRect.size.width / currentViewSize.width;
                    widthScaling = currentViewSize.height / insetRect.size.height;
                    heightScaling = currentViewSize.width / insetRect.size.width;
                }; break;
            }
            
            imageVertices[0] = -widthScaling;
            imageVertices[1] = -heightScaling;
            imageVertices[2] = widthScaling;
            imageVertices[3] = -heightScaling;
            imageVertices[4] = -widthScaling;
            imageVertices[5] = heightScaling;
            imageVertices[6] = widthScaling;
            imageVertices[7] = heightScaling;
        });
    });
    
//    static const GLfloat imageVertices[] = {
//        -1.0f, -1.0f,
//        1.0f, -1.0f,
//        -1.0f,  1.0f,
//        1.0f,  1.0f,
//    };
}

Xcode11和iOS13之后，系统严格要求在子线程中做UI操作。
说明：卡顿只会在调试模式有问题，上线后不会卡顿。