Hi, I have great web maping experience.
You are using Openlayers 2 which uses Div for drawing that is why it is taking too much time.
I will convert your map code in Opwnlayers 3 which uses canvas for drawing and it is blazing fast than Openlayers 3.
I recently make and application in which I used Openlayers 3, If you are interested ping me, I will show you a video demo also.