What a great start to the challenge! We sent out about ten times the normal amount of tasks in the first day, which is fairly typical for challenges.

I am surprised, that number of pending units raise, regardless 10 times more WU sent. I was expected faster validation time.

For the challenge people are running the AP tasks on hardware that they normally wouldn't use for AP. Some of that hardware being slower CPUs that take a long time to run units.
Normally I run a GTX 1060 (40 minutes a task) and a GTX 760 (2.5 hours a task). Now I also have an i7-4790 (16 hours a task) an Athlon X4 635 (48 hours a task) and a Pentium dual core E5500 (also 48 hours a task) running for the challenge. So you would be waiting a long time for tasks to validate with slow hardware like that running.

btw:

Hans Ivar Riesel (born May 28, 1929 in Stockholm, died December 21, 2014)

https://en.wikipedia.org/wiki/Hans_Riesel

There was a thread Hans Ivar Riesel passed away last year. /JeppeSN

One more interesting thing.
x6 1055T @3,2 GHz 4Gb (2x2Gb) DDR3 1600MHz -1WU/66 hours
x6 1055T @3,6 GHz 4 Gb (1x4Gb) DDR3 1700MHz - 83% of 1 WU/95 hours
x6 1045T @3,1 GHz 2 Gb (1x2Gb) DDR3 1550MHz - 51%/79 hours

That's insane difference between 1 and 3. More than 2 times.