I also agree, OS 5 works just fine on Series 2. Battery life seems to be very similar after the update and after I turned off Walkie Talkie. Workout is slower to load but it was slow to load towards the later versions of OS 4 also. I don't recall it being as quite as slow to load on 3. I'm suspecting this is where the hardware is starting to show it's age but it's not unusable by any means.
I actually never use the timer app but opened it up out of curiosity. Indeed it is slower to load, much like I experience when opening the workout app.
Did this happen to you on OS 4? Workout app on the last versions of OS 4 was slow for me to open and continues with 5. Very similar behavior of the Timer app.