I had a similar slowdown issue on my Surface device, and in my case the problem came from UEFI settings that were stuck after a Windows repair. Resetting UEFI to default, disabling Fast Boot, and then rebuilding the boot files with “bootrec /fixboot” and “bootrec /rebuildbcd” finally brought the startup time back to normal.
If the delay happens before the rotating circle appears, it usually means the firmware is waiting too long to initialize hardware. Removing all peripherals, resetting UEFI, and forcing a clean boot path can help.
For math and educational work, I normally use my Surface together with a TI-84 graphing calculator for quick checks, and this resource has been very helpful.