My Nexus 7 (2013) is much slower these days. I don't know if it's related to Android M but it feels slower, for sure. I did try wiping the cache which helped for a while. And I've found reverting to the stock launcher helps make it feel snappier along with a reset of the device. I'm tempted to put Cyanogenmod on it although more out of interest than anything.
However it's now a backup family tablet for streaming as I bought an Nvidia Shield earlier this year. The Shield has been a fantastic device and has a few noticeable benefits over the Nexus 7 including less reported ones like it's headphone output which is much better. Brilliant for late night Netflix watching.
The shield is a great Nexus 7 replacement even if you're not a gamer. Software updates are fairly regular atm, although not monthly like my Nexus 6, and I'm not sure the Shield is going to get Android N. Battery life is OK (about 5 hrs screen time) but not stellar. Performance wise it is very fast and responsive though. Shame that it has gone up in price but unavoidable.