Is there a way to filter out low CPC ads?
The most important thing is to understand how AdSense prices are set. AdSense, as with most ad networks, is auction based. The ad that appears on your page is the ad that bid the highest. The prices are set by supply and demand, more supply for less demand == lower price, and higher demand for the same supply == higher prices.
One can filter out out "low CPC" ads but the question to ask is what will replace them. In an auction system they will be replaced with the next highest bid, which is priced lower than the "low CPC" ad in the short run. In the long you are reducing demand, by blocking bidders and you are keeping supply the same so as a result prices will fall.
Another option is not to replace the ad, simply block out the slot. This will keep supply and demand equal and should not negatively impact prices, but you will have blank ad spaces and be forgoing potential ad clicks albeit low priced clicks. (See the ad-balancer feature in AdSense)
A third option is to set a price floor, block AdSense ads from showing below a certain price and replacing those ads with ads from another source. But this is not possible with AdSense alone, you would need to implement Ad Manager or another ad serving platform. The other issue here is finding other ads to display, you either need to sell direct, or find another ad network. Optimally managing multiple ad networks can be challenging. You could also usse in house ads to promote your own content.
The final option, is header bidding, where you get multiple ad networks to compete against each other. This essentially maximizes demand for your set supply, thus maximizing price. But it can be complex to set up and requires you working with a third party, such as AdExchange and also requires the AdManager. But given sufficient volume this is the best solution.
One other point, I don't focus on CPC I rather focus on RPM, there are several ways to get paid by ads CPM, avCPM, Engagement, and clicks, ultimately I don't care which method the user uses, what I care about is that I am maximizing revenue. RPM aggregates all these methods and gives you one number, so I worry only about RPM.
Is it a matter of time?
Yes, to some extent time does have an impact. Higher demand drives higher prices, if advertisers are not aware of your website they may not be targeting your audience. As your website get known, as advertiser begin to see good results from your website, they may begin to search you out specifically which equals more demand. How big is this impact? It is really hard to say as it really depends on your niche and demographic, and it is difficult to measure.
