Slow responses do have an effect. The average response time is just that. So, the effect can be small if they have a lot of offers otherwise (given how averages work). The bad effect is capped at 7 days (not 3). I can look into someone's response time score, if you like. I have a tool that shows all the numbers that go into the average.
I agree it's inconsiderate not to respond to offers. I disagree that it's more often veteran traders. I know of a few like that, but most are very good at responding.
I don't think setting the expiration to 90 days is a good idea. That just puts you in limbo longer. You may want to set it to something shorter (2 days instead of the default 3) instead. If someone doesn't respond, it very likely means they are not interested.
I think NathanOfLight makes a good point. Just move on and send out offers to other people. From what I've seen, the shotgun effect is the most effective (traders who have done this make a lot more trades from what I've seen over the years). Also, just the attitude of not worrying about it or getting attached to a specific offer/match/trades if you don't get a response helps a lot.
I absolutely agree it sucks when someone doesn't respond. It's not at all a new problem. This has gotten reported many times in the past. We've tried some things (e.g. the response time was added at some point, I've talked to the people who do it, there have been long discussion here about it). But there's no reasonable solution as far as I can tell. Thus, the best thing is just to assume they declined if they let it expire.
> Has anyone else run into this behavior from traders?
Tons of people. It's a well known problem on GameTZ and has been reported for over a decade if not more.
> Is the time that the offer is ignored part of the "offer response time" calculation?
I'm not sure I follow this (not sure what you mean by "ignored"), but I think the answer is yes. The time is based on when the offer was originally created until the other traders makes some response (counter/message/decline/etc) to the offer. It doesn't count anything after that so if there's some back and forth with counters, that's not part of it.
I can look into individual cases, if you like (e.g. the effect an offer you sent may have had). I think the effect only becomes part of the average once the offer is resolved (so if you're still waiting on one of those 90s, it may not have impacted yet ... and the effect is capped at 7 days).
But, just to emphasize the point again, getting all bent out of shape over this (which is understandable), doesn't do much good for anyone. If you can refocus on making other trades with other traders, you'll be better off in the end. Leave expiration at 3 days or consider making it 2 days. When it expires, take it as a decline and move on.
A very common reason for lack of response is that people dislike saying "no" to someone (and don't want to haggle over something they're not interested in). Send out lots of offers and don't get too attached. Use the forum as HAMMER points out, many say it's the best place to make trades.