i installed windows 10 technical preview to my Lumia 625.after restarting start up screen not available, it shows loading for a long time and start screen not coming. why
This happened to a lot of people and there is no clearer answer then "these things can happen when you use a TP".
But often a hard reset can improve the stability of the TP but that also does not work always.
So have a look at the TP and use the windows insider app to give feedback and decide if you want to stay on the TP or roll back.
But during the update the phone has to do a lot of "work" and a phone with a better SoC can do it faster but that does not mean that a phone with a lesser SoC can not do it.
It just takes more time.
I've got it running on a 625 also - agree it's slow and not stable - though that's expected. That said it's entirely possible! Whether you want to over something with a newer processor and/or more ram is another question.