Have you ever cleaned the fan?
Data files and photos won't slow down start up. It's the amount of programs you have that start up automtically on boot up that causes the probem.
Very good advice here:
http://www.techradar....ta-boot-faster-605430
Acer has an easy to use 'restore to factory settings' facility that you can use as a last resort. You need to be aware the computer will then be exactly as you bought it, so you will need to back up your files, photos, music, everything first.
It is daunting but I like to do it every two or three years because it really does make the machine run faster. Be prepared to spend a few hours updating, installing your programs etc afterwards.