There is a known issue with Xcode 15 that results in slow test execution, causing a roughly 25% slowdown compared to Xcode 14.3. To mitigate this problem, Bitrise offers several measures such as using an open-source tool called yeetd, which can improve simulator performance by killing unnecessary processes. Additionally, using faster hardware like the M1 Max machine with substantial CPU and memory resources can also speed up test execution. Customers can keep an eye on stacks.bitrise.io for updates on MacOS and Linux stacks, including the availability of Xcode 15.1 with potential fixes for this issue.