The summary.
ESTABLISH WHETHER YOU ARE PROCESSOR OR GRAPHICS LIMITED FIRST
Reduce resolution: if frame time improves, it is the graphics processor. Optimising the wrong one changes nothing.
Frame spikes matter more than the average — they are perceived as stutter, which is more objectionable than a steady lower rate.
PROFILE ON THE WEAKEST DEVICE YOU SUPPORT
A development machine conceals every performance problem.
MOBILE THROTTLES THERMALLY, SO TEST SUSTAINED PLAY
A game running well for two minutes may run badly after ten. And download size determines whether people install at all on metered connections.
POOL OBJECTS AND LOAD ASYNCHRONOUSLY
Allocation, collection and mid-play loading all produce spikes.
BUILD REMAPPING AND TEXT SCALING IN EARLY
They are difficult to retrofit, and remapping accommodates needs you cannot anticipate. Never encode information in colour alone.
PLAN LOCALISATION FROM THE START EVEN IF ONLY ENGLISH SHIPS
Never join sentence fragments — word order differs between languages, and retrofitting costs far more than designing for it.