Andras and I did some more digging into this issue.
Logged tccd using this command:
log stream --debug --predicate ‘subsystem == “com.apple.TCC”’
During startup, there is a 50sec window where tccd is presumably checking the application signature:
2019-11-13 12:41:30.733517-0500 0x7694f Info 0xa9379 199 0 tccd: [com.apple.TCC:access] adhoc signed StaticCode :0x7fa6d33950e0 START
2019-11-13 12:42:23.224389-0500 0x7694f Info 0xa9379 199 0 tccd: [com.apple.TCC:access] adhoc signed StaticCode :0x7fa6d33950e0 END
This seems to be the main cause of the hang in MacOS 10.14 for many unsigned applications.
Creating an ad-hoc signing with the following command allows Slicer to open without the delay:
codesign --force --deep -s - /Applications/Slicer.app/Contents/MacOS/Slicer