The other day I was going from King's Cross to London Bridge. I was on
my way down to the tube, but as I passed the Thameslink platforms
there was a southbound train sitting there, so I jumped on that on the
assumption it would be quicker.

It wasn't. At all.

When TL2000 is finished, will such journies actually be quicker? Or
will Thameslink trains continue to trundle through the city at roughly
walking pace?

Jonn