"Three 40 second glows" is a LOT! That must have been on a very very very cold day. That is way outside the recommendations of MB that came with our cars, so I wonder if something else was going on. After I replaced my GPs the first time (as a preventative measure upon obtaining the car), I noticed that the required glow time was much longer. It turns out that there was so much caked on dirt/oil on all the connectors that my "fix" was actually worse than before. I lived with it until the Bosch plugs started suffering premature failure due to my engine running too hot. So I took it all apart again and used sand paper and steel wool to clean everything. I also used antioxident grease on all the connections. This is the grease you are supposed to use for aluminum wiring in homes (kind of the opposite of dielectric grease).

If I didn't know better, it would be easy to conclude that my new Beru plugs work way faster than my old Boschs. But it is just that I got everything cleaned up appropriately. Your experience of going from 3X40 seconds to 10 seconds is remarkable. My w124 probably needs 15 seconds of glow to start without too many misfires. And my w115 probably needs 30 seconds of glowing. These numbers are on a cool San Luis Obispo morning, which is probably 45 deg F. The compression on the w115 is good (400 psi or more), but I haven't checked the w124.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).