Performance is something we are constantly working on improving. Marvin is a complex app and depending on how you are using it there are a lot of calculations being run in the background that can slow things down.
Here are some tips on what you can do to try and speed things up.
The limiting factor in how quickly Marvin can run is your free memory on your computer. If you haven't restarted your computer in a while, are running other applications or have lots of tabs open... all these things take up memory on your computer, slowing things down.
Restart & Close
So the first measure if Marvin feels slow is to restart your computer. You also might want to make sure you don't have any applications open that you are not using. And closing tabs might further help.
Deleting unused Smart Lists
If you have a lot of Smart Lists that you no longer use, delete them.
Turning off the task count
One of the main things slowing things down is having to calculate the task count for a list. If you are using the custom sidebar you can disable the task count for each list.
You also want to make sure the counts are not showing for the collapsable default lists (categories, labels, smart lists, tags). Uncheck this box at the very bottom: