So just to update recent IME and cutting to the chase: based on my recent experience I would recommend 2 hours minimum for connections.

Our LHR-FRA flight had a delayed arrival of 30m or so, which shaved our connection time to approx 1.5 hours. We arrived at a Z gates and then dutifully followed the signs which unfortunately took us outside security and ultimately to a train stop that was not operational. However we quickly figured out there was another track which required use of stairs. At no time did we see any staff to assist until we got to B. Upon arrival at the B terminal we endured security screening which - much to my surprise - was very fast! Maybe a 5 minute line, though I was delayed another 5 min to clear one of my bags. Fortunately the hike to B15 was not far. Made the mistake of taking the escalator to a Senator lounge which looked as if it had been closed for some time (no indication of closure on the main floor). Fortunately there was a special lounge above B14 that had the beloved pretzels.

Overall, the security was much faster than expected - but the signage and dead ends were disappointing.