There are several reasons why thinkorswim (TOS) may be running slowly, including a processor that can't provide enough horsepower, or a very slow or intermittent internet connection. If you've ruled those out, since other things run plenty fast enough, then it might be that thinkorswim's memory is limited. Some symptoms of memory limitation in thinkorswim include responding very poorly, where it's running slowly, being way behind in responses to your mouse clicks, or being unable to open additional windows. If thinkorswim doesn't have enough memory to do certain operations, it might have to collect unused memory and struggle to gather enough free memory to do things, slowing it to a crawl. Increasing the available memory available may help it run faster if it's no longer limited this way. Here's how to adjust it.
When you launch thinkorswim, at the point it asks you for your username and password, you can click on the settings gear in the lower left corner. You can see the gear pointed to by the red arrow in the following image:
You'll see a setting screen appear where you can adjust several different things within TOS, including Font Size, and more. At the bottom of the settings are two pulldowns relating to Memory usage, as shown in the image below. The left pulldown indicates how much memory thinkorswim should start with. The right pulldown selects how many Megabytes of memory thinkorswim can expand to use. If you have a lot of memory (RAM) in your computer that you're not fully using, feel free to increase the max value a bit larger to see if it improves performance. In the image below, the 12 MB selection is being made. Your installation may offer different values. You can experiment with different values to see how it performs. The left pulldown determines how much it should start with, so if you set the right one to 12 MB, you could set the left one to the same so it has all it needs without having to expand, but feel free to keep the starting value lower and it will obtain more if it needs it, up to the right pulldown's limit. Click the save button to accept the new selections and continue with the login prompt.
If you want to better understand how much memory thinkorswim needs, which can vary based on many things, including how many windows you have open within thinkorswim, you can use the main panel's Help / System tab to see what the memory usage is. At the top of that screen, you'll see three values about memory utilization. The rightmost Free Memory indicates how much memory the application has available to do things. If that gets near zero, then you're likely limiting thinkorswim, and could benefit from an increase in allocated memory. If it's a very large number, then you might be able to reduce the amount of memory that's allocated to thinkorswim.
One other way to gauge whether you're using the correct amount of memory is to see what the overall memory usage is in your computer. In Windows, you can check to see how much free memory is available across your whole computer using the Task Manager's Performance tab. Once in task manager, click on the Memory button at left to view memory utilization (click on the More Details if you don't see multiple tabs across the top). You may see something similar to the image below, where about half of the available 23 GB of memory is being used. If you have a lot of unused memory, this would mean increasing the amount of memory available to thinkorswim would be ok. If instead you're using almost all of your available memory in the computer, then perhaps you would want to reduce the amount of memory allocated to TOS. Usually, this is not something you need to worry about, if you use a smaller number for thinkorswim's "left" memory pulldown (as described above), and a larger number for the "right" memory pulldown.