I'm looking to upgrade to 802.11ac on my home wifi network, and have noticed that most airplay certified wireless speakers only contain 802.11g chips. Will including one or more of these 11g devices cause problems or slowdowns with the 802.11ac data transfers? (Assuming 802.11ac router and 802.11ac laptop)
Well, first off, unlike 802.11n which could work in either the 2.4GHz or the 5GHz band, 802.11ac is defined for the 5GHz band only. Note that 802.11g was 2.4GHz only. So you'll never have 802.11g and 802.11ac devices sharing airtime on the same radio frequencies. All of the 802.11ac APs I'm aware of on the market so far are simultaneous dual-band devices, supporting b/g/n in 2.4GHz, and a/n/ac on 5GHz.
Also, despite popular and widespread misconceptions, the presence of older-generation wireless clients on the network DOES NOT, AND NEVER HAS forced the whole network down to older-generation rates.
So let's say this was a question about whether your G AirPlay device would bring down the speed of your 2.4GHz 802.11n devices. When it's your N client's turn to send/receive data to/from the N AP, it will still do it at N rates (if it's within range where N rates are the best choice), even if there are G clients on the network.
However, when it's the G client's turn to send or receive data, that client can only do it at G rates, so it's likely to use up more airtime on average to transfer the same amount of data. That leaves the AC clients with less airtime to work with, but they're still able to transfer stuff at AC rates when they get airtime, so unless the G client is doing so much work that it's taking up all the airtime, the AC client's wont be impacted that much.
Let's do a back-of-the-envelope estimation to see how your G AirPlay device might affect your 2.4GHz b/g/n network. In the case of AirPlay speakers, you're talking about an audio stream that's perhaps as small as 256kbps with lossy compression, or 1.4mbps completely uncompressed. 802.11g devices, in real-world conditions, often get 15mbps of TCP throughput (because they don't always get their top 54mbps PHY rate, and because Wi-Fi is half-duplex and has a lot of protocol overhead). So while streaming audio to your AirPlay speakers, you might be using from 1/60th to 1/10th of your air time. So even in the worse case estimate, your N clients still get 90% of the air time to use at N rates, so they should still be able to get 90% of the speed they would have gotten if the G client hadn't been there or hadn't been doing anything. That's a lot better than the mythical "it will force them to use G rates", which would have meant cutting them down from a max PHY rate of 450 to a max PHY rate of 54mbps, which would have been a 90% reduction compared to a 10% reduction.