How I tested this:
1. Created empty project
2. Added scripts for testing (attached below)
3. Entered and exited the play mode 5 times
4. Recorded data

Results when compared to the best performing (out of supported version) 2017.4.29f1:
- 2019.3.0a7 is 80%-100% slower
- 2019.2.0b7 is only ~18% slower (notice that 2019.2.0a1 was 71% slower)
- 2019.1.8f1 is 50% -80% slower
- 2018.4.3f1 is ~ 40% slower
- 2017.4.29f1 is a baseline
- 2017.2.5f1 is 10- 15% slower

Note:
- Tested on two different Windows 10 machines