Having a root/admin account for doing installs and maintenance tasks and a lower privilege account for day-to-day work has been the standard model for a long, long time. It has many practical and security advantages. The disadvantage is that it is inconvenient.
Whoever told you it slows the computer down were either confused, making a big generalisation or just simply talking rubbish and I can only imagine they were referring to user accounts running concurrently. If you had 10 user accounts logged in running 1 program in each, it wouldn't be that different from running 10 instances of the program under 1 account. There would be a little overhead involved in the former and you'd use a bit more memory, but it would be a big stretch to generally say user accounts "slow the computer down".