If it’s so bad that your phone is literally heating up by having the app open, it points to an infinitely loop within the app. The only issue is I cannot reproduce anything like it so this infinitely loop must be some rare case. I just have no idea what the case is :/
One other thing. It looks like Graphene OS does not come pre-installed with Google Play Services. Summit uses Crashlytics for crash reporting which is part of Firebase which uses Google Play Services. I wonder if this is causing issues. You can also try disabling Firebase within the app. To do so go to Settings > Logging > Log crashes to Google Firebase. Let me know if this helps.