how can i change the default style to product image thumb scroll gallery?

Hi there,

I'm not sure unfortunately. I'd assume you will need to find a plugin for it.

That is all handled by WooCommerce plugin itself so it might be worth checking with their support team to see if they have any suggestions when there are that many variant images per product.

I have tried some plugins, the problem is all of them will slow down the page speed obviously.

Unfortunately that's the nature of sliders - they tend to slow down the page speed a bit.