This isn't a github runner issue, it's an Azure VM issue. They use Azure VM SKUs with temp disks, and windows VMs on those SKUs by default spin up with C on a remote-backed, persistent file share and D on the local temp disk. The remote-backed file share OS disk is absolutely slow, especially if they're using Standard HDD or Standard SSD disks. You can spin up your own VMs that use Premium OS disks for slightly better performance, but if you are doing anything serious on an azure VM, you should use the temp drive (which, by the way, mounts to /mnt on Linux) or ephemeral OS disks.

(And also this is all for v5 and earlier skus and changes slightly for v6 skus but whatever).