Andrea:

There is a first slowdown at the first wipe transition, then slowly
the playback resumes fluidity. At the second transition the insertion
point slows down a lot, so much so that it seems blocked, while the
audio continues to flow normally.
...
I don't know when this problem started, but no less than a month. I
don't even know if it's due to my PC. Surely it had never occurred
before.

It is not obvious what the problem is.  I watched your video and can see it slow down, but I can not create the same situation here.  I ran a couple of tests using yesterday's build versus December 30, 2018 build with 10+ transitions at the edits and do not see any problems.  Attached shows the tracks with the transitions in it that I played over and over and timed using:
  time ./cin
Results were as follows:

Build date - 12/30/2018
   Test 1                       Test 2
Session time: 0:01:54   Session time: 0:01:48
real    1m54.902s       real    1m48.314s
user    0m59.660s       user    0m59.383s
sys     0m5.677s        sys     0m5.601s

Build date - 02/04/2019
   Test 1                       Test 2
Session time: 0:01:52   Session time: 0:01:48
real    1m52.613s       real    1m47.953s
user    0m59.356s       user    0m59.005s
sys     0m5.511s        sys     0m5.662s

I am going to try a more complicated test yet and compare.   Phyllis