onTapCanvas: function (event) {

console.log("onTapCanvas ", event);

let that = this

let canvasX = event.detail.x - event.currentTarget.offsetLeft

let canvasY = event.detail.y - event.currentTarget.offsetTop

let centerX = canvasX - size * 0.5

let centerY = canvasY - size * 0.5

let ctx = that.canvasContext

let path = “…/…/images/SH04-CO00.jpg”

ctx.drawImage(path, centerX, centerY, 64, 64)

ctx.draw(true, function () {

})

},

这样一段代码，手动在画布上进行单词点击绘制图片，进行这样的操作多次之后，每次绘制的帧率越来越慢，从刚开50fps一直降到20多，看到图片绘制到画布的反馈越来越慢。

注：ctx.draw中true参数是为了保留之前绘制的内容。

请问是什么原因造成这个效率下降的？