There are some guides like fatty foods (e.g. chocolate and pizza) tend to be slower release but others may be a bit of a surprise.
Have a read about "Glycaemic Index" - this is the measure of how fast carbs are absorbed.
We all react slightly different so, if you want to find out what is slow for you (rather than "in general"), trialling a Libre is very useful. Unfortunately, it is difficult to get this on the NHS but even one Libre, which lasts for 2 weeks, can provide excellent data if you use it as more than a finger prick replacement.
The cheapest option for purchasing the Libre in the UK, at the moment, seems to be Asda where they sell them for £44 each.