Hi, I thought I'd share my experience with upgrading to 19.07.2 and to ath79 target from ath71xx.
I have TP-Link Archer C7v2, which is my main router and connected to VDSL-modem, and Netgear WNDR3700v1 as a secondary router. I have set up a wireless bridge using WDS over 5G wifi between them to extend the network to another room with wired clients connected to secondary router. This setup was working perfectly fine for me with OpenWRT 18.06.1 for a long time and I could easily saturate my 100/10 Mpbs VDSL over the bridge and latency was fine.
Last week I decided it was time to upgrade and I flashed both routers with 19.07.2 ath79 target. I replicated my config by hand, as it's not compicated (did not save configuration with sysupgrade). After the update with the same exact setup, ie running 5G WDS I was now getting at max 40Mbps over the link and it seemed that ping was also more erratic. I tried to change Archer C7 ath10k drivers to basic version from -ct, which had no effect. I also tried running the link over 2.4G wifi and it was actually a bit faster than 5G, but still below expected.
I still don't know which one of the routers started behaving badly with the update, but clearly something is not working as it should with these routers with the new release. Finally I decided to downgrade to 18.06.8 and now things are back to normal and speed is back up.
So if you are having wifi speed problems with 19.07.2 on these routers, downgrading to 18.06.8 and ath71xx might help.
BR, PP