So I suggest first to look on which features were added from UEX 15.1 to 16.1 to find out if there is a new feature which could make opening a file of ... (encoding, size, type, ...) slower in UEX 16.1 in comparison to UEX 15.1 and perhaps not needed by you. One main difference is themes support. The themed GUI makes starting UEX 16.x definitely slower than starting UEX 15.x with no themes support. It might make also opening a file in UEX a bit slower. I know that from UE for Windows where I'm using a very "puristic" theme to increase display updates dramatically in comparison to standard "stylish" themes.
My second suggestion is compressing a file opened by you and your entire configuration - I think stored in ~/.idm/uex/ - into a ZIP file and send this archive file with additional information (steps to reproduce, how you measured loading time, your exact OS version, etc.) to IDM support by email for evaluation of opening time by IDM using UEX 15.1 and 16.1.
Best regards from an UC/UE/UES for Windows user from Austria