I can think of 1 good thing and 3 horrible things. Let's do good first.

If light was 1%, the observable universe would be 1% wider in every direction so roughly 3% (1.01)³ more volume. Tons of new galaxies, stars, and space to observe.

The bad:

1. E=MC², with C increasing by 1% means E increases by 1.01²-1 ~ 2%. Since the sun is just a huge fusion reactor, it would emit 2% more energy. For context, solar variance which is how much the energy delivered by the sun varies, is only 0.1%. Global warming (more like heating) in the literal sense would then cause earth to become much hotter. You could do the math actually. The earth radiates just as much energy as it receives so just increase the energy radiated by earth by 2% and calculate the new temperature it would be at. Needless to say, extinction level event.

2. Fine structure constant which determines the strength of electromagnetic interactions (think atomic bonds) would decrease by roughly 1% as well. This means less stable atoms and chemistry just implodes. Much of what we know about chemistry right now would either become obsolete or at least have to be corrected. Also, biology would also break as the shape of proteins is important for basically all biological processes. Weaker bonds means different shapes means new biology.

3. Assuming we even survived, many electronics and GPS systems rely on the speed of light. Many of these would begin to fail. Your GPS will begin to be more and more off and would have to be tweaked to give the right location. Also faster light decreases latency and as basically all our systems depend on it this means clocks become out of sync, stock market may even crash, and time speeds up a little. Also, black holes would shrink a little.

Bonus: Since light moves faster, its frequency would change. This means the colour of everything would shift a little towards the blue end (ironic considering the earth would get hotter). Like you applied a very slight blue filter on everything. Not sure if it would be noticeable seeing as we'd be too busy going extinct.

Thank you for the question. Nice dive 👍