GridView 加载缓慢

我正在尝试实现一个显示资产照片的 GridView。一切正常，但滚动时，图片出现延迟。你会有什么建议和建议？

inner class ImageAdapter(private val mContext: Context) : BaseAdapter() {
        private val am: AssetManager = mContext.assets
        private val files: Array<String>? = am.list("img")

        override fun getCount(): Int {
            return files!!.size
        }

        override fun getItem(position: Int): Any? {
            return null
        }

        override fun getItemId(position: Int): Long {
            return 0
        }

        override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
            var convertView = convertView
            if (convertView == null) {
                val layoutInflater = LayoutInflater.from(mContext)
                convertView = layoutInflater.inflate(R.layout.choose_element, null)
            }
            val imageView = convertView!!.findViewById<ImageView>(R.id.gridImageview)
            imageView.post {
                object : AsyncTask<Void?, Void?, Void?>() {
                    private var bitmap: Bitmap? = null
                    override fun onPostExecute(aVoid: Void?) {
                        super.onPostExecute(aVoid)
                        imageView.setImageBitmap(bitmap)
                    }

                    override fun doInBackground(vararg p0: Void?): Void? {
                        if (!listId.contains(position)) {
                            bitmap = getPicFromAsset(imageView, files!![position])
                            list.add(bitmap!!)
                            listId.add(position)
                        } else {
                            bitmap = list[position]
                        }
                        return null
                    }
                }.execute()
            }
            return convertView
        }

        private fun getPicFromAsset(imageView: ImageView, assetName: String): Bitmap? {
            val targetW = imageView.width
            val targetH = imageView.height
            return if (targetW == 0 || targetH == 0) {
                null
            } else try {
                val `is` = am.open("img/$assetName")
                val bmOptions = BitmapFactory.Options()
                bmOptions.inJustDecodeBounds = true
                BitmapFactory.decodeStream(`is`, Rect(-1, -1, -1, -1), bmOptions)
                val photoW = bmOptions.outWidth
                val photoH = bmOptions.outHeight
                val scaleFactor = (photoW / targetW).coerceAtMost(photoH / targetH)
                `is`.reset()
                bmOptions.inJustDecodeBounds = false
                bmOptions.inSampleSize = scaleFactor
                bmOptions.inPurgeable = true
                BitmapFactory.decodeStream(`is`, Rect(-1, -1, -1, -1), bmOptions)
            } catch (e: IOException) {
                FirebaseCrash.report(e); // Generate report
                e.printStackTrace()
                null
            }
        }
    }