Somewhat similar problems on my late 2013 iMac too. Photos was working OK until upgrading to High Sierra. Then it started crashing regularly. It could take anything from a few seconds to a few minutes before it crashed again. It seemed to struggle most with cropping, but could crash anywhere, anytime. Many, many crashes. Almost not possible to use for editing. Complete re-instal of High Sierra made no difference.
Sometimes it spontaneously closed, sometimes it would lock up, and sometimes it would cause the whole computer to go back to the log-in screen. And on occasion the screen would flicker or break up in various unusual ways.
It looked like it might possibly be a problem with the graphics drivers (the computer came with Nvidia GeForce GT 750M graphics) so I downloaded some High Sierra compatible drivers from Nvidia. Problem Fixed! Photos working again. Whoopee!
Then Apple put out an update and "bug fix" for High Sierra. When installed it rendered the Nvidia drivers no longer compatible and installed Apple ones instead. Problems back again! Nice work Apple...
Fortunately, Nvidia have kept pace and also quickly released newer drivers which are compatible with the latest version of High Sierra. When I install them Photos worked again.
Given that the Nvidia graphics are Apple original equipment, you'd hope that they'd talk to each other and get it sorted before putting out "upgrades". Ah well, it's working again... for now.... Fingers crossed...