Sauna heating Questions

I treated myself to a barrel sauna without many frills. It’s a small model, measuring about 5.2 feet in depth and approximately 6.6 feet in diameter. I assumed a 6 kW heater would be sufficient. I ended up purchasing the HUUM Drop 6 kW heater along with the UKU control system. Initially, the sauna reached temperatures of around 158°F after 1.5 hours, which I found pretty underwhelming. I then completely redid the sealing on the glass door, and voilà—after 1.5 hours, I was able to achieve temperatures of around 207°F. The outdoor temperature was always around 32°F. Is it normal for it to take that long to heat up?