Will the Monero GUI-wallet be as slow to sync as the simplewallet is?
If it will, will there be a light version of GUI available like bitcoin has Electrum?
GUI uses the "normal" daemon to sync, so I don't think there will be significant difference in syncing times. However, you could connect the GUI to a remote node, which kind of results in a light version then.
Will the GUI support viewonlywallets?
Likely later, when more advanced settings are visualized.
Does the using of SSD as hard drive, make any wallet faster to sync? Or is it more depending about the processor?
How soon is soon?
Yes, that makes a significant impact. An SSD is probably 3-5 times as fast sync wise than an HDD. Sync time also depends on the processor, but more so on kind of hard drive.
Probably with the next tagged release.