You haven't given much detail about how much of an increase in render times you're experiencing, so it's hard to know if this is abnormal.
Note that evaluating expressions takes some time, and it's common for the use of expressions to add considerably to render times.
Also note that the use of expressions can force After Effects to render using only one process, even if you've enabled Render Multiple Frames Simultaneously multiprocessing. Pay attention to the Info panel for helpful messages that tell you when this is occurring.
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Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
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