It's a chicken-and-egg problem, but it's starting to improve.
Honestly if google really wanted to push this, they would do something to penalize http only ads. That would really pressure advertisers to upgrade or lose.
Why don't ad networks care about HTTPS? It would increase the number of potential sites that can show their ads, make it more difficult for ISPs to block their ads, and allow browsers to load their ads over HTTP/2.
If slow ads were a problem, why are ad networks so damn slow? I frequently see ads taking 10–20 seconds to load on major news sites. Serving fast ads would mean more viewing time, yet that doesn't seem to be a priority for anyone.
"On our production frontend machines, SSL/TLS accounts for less than 1% of the CPU load, less than 10 KB of memory per connection and less than 2% of network overhead. Many people believe that SSL/TLS takes a lot of CPU time and we hope the preceding numbers will help to dispel that."
Honestly if google really wanted to push this, they would do something to penalize http only ads. That would really pressure advertisers to upgrade or lose.