Test your device's approximate disk I/O performance via the browser's IndexedDB API.
This test generates a 50MB file in your browser's local database and measures the time it takes to save (write) and load (read) it.
This is an approximate browser-based estimate. Due to security sandboxing, browsers add significant overhead to file operations. These speeds represent what web applications can achieve, but will be noticeably lower than your disk's true raw hardware speed.