UPDATE: Got the hose put back in about 400 miles ago, same highway, speeds, ambient air temps, and fuel. Gas mileage 11% better, and car seems friskier. Unfortunately, found another part clunking around loose in the area behind the headlight; mech swears he never saw it or did anything with it. It's a complex molded black plastic part about4" by 2" overall- will try to sketch it and post the sketch to see if anybody knows what it is. Wonder if the guy who bought my '40 Packard will trade it back for the Volvo?

I highly doubt that that air tube could possibly have much, if any effect on anything noticeable. All it does is bring in (possibly) cleaner air from the front of the car and heat shouldn't normally be a problem above the trans (bottom of aircleaner) as fresh air is still circulating there as you're driving.
A few years back in the "good ol' days" when cars used carborators, most drew their air from under the hood w/o any problems. OK, in the 70's domestic cars then added fresh air hoses to help keep the filters cleaner but when the cars aged and those tubes fell apart, then discarded, nothing changed as far as drivebilty was concerned.

"Could this be a contributor to the car being somewhat sluggish?"

Absolutely. A couple of weeks ago, my fuel consumption increased drastically and my car was running as it didn't want to. When I got ready to clean the throttle body, being directed as that might be the cause, I've seen the same thing as you did, the only difference being mine was disconnected from there. just plugged it in, and the spirit came back. I wouldn't even imagine that such a minor thing to make that drastic change.
HAppy driving.
Ihsan