You can specify “parallel” mode for a Test Suite Collection. Katalon Studio free version provides it.
You want to create a Test Suite Collection which comprises with multiple Test Suites (TS1, TS2, TS3,…) that contain one or more Test Cases. If you specify “parallel”, then TS1, TS2, TS3 will be executed in parallel.
If you find your test cases run slowly, then running them in parallel will make your test project even worse. Parallelism would not solve the slowness at all. You should make efforts to shape your test cases run as fast as possible.
Do you use WebUI.delay(n) with fixed value of n? That makes your test case run slow due to unnecessarily long delays. If yes, you should avoid it. You should prefer WebUI.waitForXXXX() keywords.
Do you find vast volume of “StepExecution” logs in the log viewer? If yes, you should disable it.
Does your Test Case script has for or while loops, or functions which are called frequently, then you should move the script out of Test Case and bring it into Keywords. In general, a Test Case script runs slower than a Keyword because Katalon Studio takes verbose execution logs of Test Cases but it doesn’t log Keywords.