RAID is not a substitute for a backup.
What you want is a backup
Yes, your HDD will hold back your SSD. Like all RAID-1's you can only write at the speed of the slowest device. Windows' can balance reads across both, but is pretty primitive at doing it.
[Edit]
Some clarification:
As per 1 and 2 RAID provides redundancy but anything other than hardware failure affecting your SSD will also be copied on to the RAID mirror (accidental deletion, corruption, viruses, etc.). It may guard you against losing hours of work if your drive dies but won't guard against losing months of work if you accidentally press "Delete".
For a typical home user scenario creating a normal backup (not RAID) from your SSD to your HDD is sufficient. For more important things, an external cloud backup can augment your system. A proper backup and version control solution will be perfectly capable of backing up every change, every second, if that's what you want.
Two SSDs in RAID + an external backup may be a hypothetical ideal solution but somewhat excessive for most users.
why is raid not suitable for back up? and losing even hours of work is painful, id rather never lose work and just re-sync if my drive dies. Don't know if tradition and old books are dictators in this day and age. Besides raid isn't back up, back ups are usually outdated as it's not possible to back up every single second. That's only possible with raid. – Surgetheurge – 2015-10-24T16:10:56.853
https://www.google.co.uk/search?q=raid%20is%20not%20backup – qasdfdsaq – 2015-10-24T20:41:53.090
Okay, I see what you mean now. So what I need is two SSDs in raid 1 + a back up system like an external disk. This way if something bad happens to the computer, the data won't die, and if something bad happens to one of the disks, then i won't lose a single second of progress. – Surgetheurge – 2015-10-25T10:14:35.927
@Surgetheurge: See my edit to the answer – qasdfdsaq – 2015-10-25T15:28:02.207