S7-316 SLOWER than S7-315

We have a few S7-316 CPUs that run 3x slower than our S7-315 CPU. They are all running roughly the same code/hardware config. Any reasons why this could happen? The 316 seems an oddity in firmware [v1.2.1 is latest] while all other CPUs can go to V2.6.
The 316 setup also has a CP 342-5 with a keypad, so I tried to delete keypad from config but that didn't help.
Would going to a 319 help?

Thanks.

Short version: your 316 is really old. Just get a newer PLC.

Long version: They stopped selling the 316 in 2004. The 315s you have are of a newer generation, and are faster with more memory. The current gen CPUs are even faster, with even more memory, and also skinnier.

315, fw 1.2: bit time .3 us, work mem 64 kB
316, fw 1.2: bit time .3 us, work mem 128 kB
315, fw 2.6: bit time .1 us, work mem 256 kB
315, fw 3.2: bit time .05 us, work mem 384 kB
319, fw 3.2: bit time .004 us, work mem 2048 kB

If you buy a current gen 315, you'll get a PLC that's 6 times faster than your 316 and with 6 times the memory. That should be more than enough. If you go to a 319, it'll be almost 100x faster, and with 32x the memory. Odds are, that's crazy overkill.

Fair warning: I've never worked with the really old 300 cpu's, and I'm not sure if there are any parts (like CP cards) that don't migrate well. However, the current gen 315 either comes with 2 profibus ports or 1 profibus & 1 profinet/ethernet, so you may not need the CP anyway.

Thanks mk42, that's what I needed to know.

yeah we recently replaced a 318, which was a powerhorse when it was released, with a 315. the only drawback was the 318 had 2 DP ports, and an extra CP card for DP installed, and one profibus network was really bad (some hand control on a really bad wire) so we couldn't join the profibus networks... A profibus switch solved that problem.
the 318 was running fine actually, but they needed network (PN) access.