Reproduction Steps:
1. Import attached project.
2. Open "Test" scene.
3. In Menu, select "Test" -> "EnterPlayMode test".

Results: In Console, the time it takes for Play Mode to begin is shown. In 2017.x versions, there seems to be a gradual uptick in load time.
Here are the results for all 5.x versions:

5.1
Time in ms: 801.242499999702
Time in seconds: 0.801242499999702
Time in ms: 797.517700001597
Time in seconds: 0.797517700001597
Time in ms: 805.629900000989
Time in seconds: 0.805629900000989

5.2
Time in ms: 836.837300002575
Time in seconds: 0.836837300002575
Time in ms: 845.467700004578
Time in seconds: 0.845467700004578
Time in ms: 853.629100002348
Time in seconds: 0.853629100002348

5.3
Time in ms: 950.551299996674
Time in seconds: 0.950551299996674
Time in ms: 831.493000000715
Time in seconds: 0.831493000000715
Time in ms: 797.730599999428
Time in seconds: 0.797730599999428

5.4
Time in ms: 819.942899994552
Time in seconds: 0.819942899994552
Time in ms: 807.845600001514
Time in seconds: 0.807845600001514
Time in ms: 808.83030000329
Time in seconds: 0.80883030000329

5.5
Time in ms: 876.640799999237
Time in seconds: 0.876640799999237
Time in ms: 828.066899999976
Time in seconds: 0.828066899999976
Time in ms: 832.803199999034
Time in seconds: 0.832803199999034

5.6
Time in ms: 913.761399999261
Time in seconds: 0.913761399999261
Time in ms: 865.965400002897
Time in seconds: 0.865965400002897
Time in ms: 871.020899996161
Time in seconds: 0.871020899996162

2017.1
Time in ms: 1054.67059999704
Time in seconds: 1.05467059999704
Time in ms: 1040.15450000018
Time in seconds: 1.04015450000018
Time in ms: 1035.89919999987
Time in seconds: 1.03589919999987

2017.2
Time in ms: 1304.16659999639
Time in seconds: 1.30416659999639
Time in ms: 1290.55640000105
Time in seconds: 1.29055640000105
Time in ms: 1270.94070000201
Time in seconds: 1.27094070000201