soc power up time too slow..

not sure if you know what device that you are using, primitive way is to disable all the unused device and feature in the preloader, also from the device tree, maybe able to speed up. and also use qspi boot instead of sd card

any way to measure the boot up time in various stages?

there is a function to read the time stamp, you may consider to use this and deploy in several important boot area, identify the most time consuming area and try to strip the initialization from there to earn faster boot time.

but you need to strip the boot up area carefully as might break some of your required feature or drivers.

could you point me any guide for this purpose?

i don't see the guide from Altera, but you can also improve this by using minimum file system