Wheels

I agree regarding wheels wont make much difference, make sure your tires are solid, get a good track pump and get them up to 100psi, you may be surprised.

It's got nothing to do with the wheels. Upgrading them won't make you noticeably faster both down and uphill. Just ride your bike.

I'd change the tyres first before the wheels. It's the cheaper option and can make a big difference. Get some Continental GP 4000s2 or similar.

Swap bikes with a friend who is a good descender and see if they go quicker on your bike so you can do a like for like comparison before splashing out.

If you are adamant you want new wheels then we're finding there's some huge discounts across the bike stores on certain wheels (maybe driven by the wiggle move?)

But honestly I'm guessing it's position related too while you get used to the bike so hold fire on any big expenditures before you make sure!

If you are new to cycling then stick with what you have for a little while. Wheels, like bikes, come in many different flavours. Some will be light, some will be stiff, some will be aerodynamic. Some will be all of these but you will need to sell a kidney to afford them.

Over the next few months work out what sort of rider you are and what terrain you like riding. This will then provide more focus to your wheel upgrade.

Don't think that during those months you are wasting your energy when riding though. All those rides on your heavier, slower, stock wheels are building your strength and endurance. When you do upgrade you should really feel a difference.

For the record i know a few riders who have the rim brake version of the Mavic Ksyrium and they are very happy with them. If the disc version is as good then you should be happy with them too.

If you have just got your bike, is the stem really high up. This increases your drag and slows you down massively compared to other riders.
The synapse does have a large headtube so you can probably go lower without giving yourself any discomfort.