The problem is that the reading indicator reads only a second and a half and I don't read the whole

It's reading way more than half a second (around 6 seconds, actually) and based on the specs of such a Mac mini everything appears to be normal. With 8 GB of memory of which only a fraction would actualyl be available to Ae you cannot expect a RAM preview of the whole timeline. You may want to read the onlien help on some of those basics:

https://helpx.adobe.com/after-effects/using/previewing.html

Mylenium