It's a database. It will depend on your PC's RAM and processing power.
iTunes may slow down as you tax the limits of your PC. Where and when that happens will be different for each user.
I have close to 12,000 song references contained within iTunes and have no problematic slowness within iTunes. It will take several seconds to read the Library database file on startup, and it takes some time to write the two files at closing.
I have seen folks here with over 15,000 references within iTunes. I do not know if there is an upper limit.