My computer running Ubuntu 16.04 has 32GB RAM, but even on heavy tasks, rarely over 8GB is consumed.
I wonder if is possible to tell Ubuntu to use more RAM to speed up the process.
Moreover, after a day of working, my computer is slowing down, and needs a reboot. Is it normal? Can't Ubuntu fresh itself up when resources are abundant?
Tell Ubuntu to use more RAM to speed up the process
Ubuntu does not control what the applications you're using do with available memory, the applications themselves do. So you'd need to find some way to tell the application you're using that it shold use more memory. However, most applications only use as much memory as they actually need - using more isn't going to improve performance. (What applications are you using, though ?)
However, what Ubuntu does do is use unused, available RAM as cache, i.e. recently accessed files are kept in memory, so that when they are needed again, they do not need to be fetched from disk. This improves performance. This is also why starting an application (e.g web browser or IDE) a second time is faster that starting it "cold", i.e. immediately after boot.
After a day of working, my computer is slowing down [...]
No, this isn't normal. I generally have weeks of uptime on my Linux systems, and performance isn't affected. Maybe check the memory / CPU usage at the end of the day, and see if something is using lots of resources. What do you mean by "is slowing down", though ?
top output if something strange is going on. Concentrate on process that use a lot of CPU and/or a lot of memory. Add this info in your question.