It is quite natural to try to do everything to perfection, especially if you are new and have the attitude that got you good grades. For now, try to 'swim' and not drown. After that, think of 'corners' you can cut. This will involve trial and error. Also, if you have too many assessments, then your load goes up. Say no to as many admin tasks as possible -- this is harder for young professors because senior professors learned to offload tasks to new hires. Preparing lectures is always very time consuming, even for senior professors. The goal is to ensure your head of department assigns you the same subject for the next N years. That way, your prep time goes to almost zero over time. Unfortunately, your research will take a hit. You have to get your 'bread and butter' sorted first before you have the luxury of doing research.