double checked for you. the Surface Laptop is using a PCIe based NVME SSD.
Depending on manufacturer of the drive itself, but PCIe NVME drives are rated for 3Gbps continuous read, about 2gbps continous rights. In line with what is standard in the industry.
they're also really cheap these days. The Samsung 970 EVO for example (capable of speeds mentioned above) can be found for approximately $100 for a 250gb version.
https://www.newegg.com/Product/Product.aspx?item=N82E16820147741
storage has come down in price dramatically. What Apple currently has listed for upgrade prices are approximately 3-5x RETAIL prices for storage. And being NVME based, are user (not easily on the sl2) replacable / upgradable.