macOS 10.14 Mojave is the only version that works properly with Encore 5.0.7 (5.0.6 1303m in About). Any Mac made between 2012–2019 can run it by creating a bootable APFS Volume (Apple has support docs on this) and installing Mojave onto it. Your 2011 does not have the Metal graphics compatible GPU required by 10.14.
“Properly” is a euphemism—“Workable” is closer to the truth. Encore over Mojave is still buggy but nothing like it is over High Sierra and earlier where certain graphics bugs cannot be avoided.
Speed is not an issue although, depending on the year and build of your Air (or other Mac), there may be much faster internal drives than came stock. I’ve upgraded many over the years.
I run Encore on a pair of 2012 MacBook Pros that I have upgraded with SATA III SSDs. My 2017 iMac Pro was the perfect Encore machine but I no longer have one. As much as I would like to use it for creating lead sheets and quick scores for my choirs, I can’t trust it to print properly—words, notes and lines sometimes disappear with minute changes to the layout. Its only use to me anymore is to open up and tweak old files.