As Shalryn hinted to, not all glass is the same.
Although I would recommend glass also, I wouldn't recommend something like a thin colored glass cup, as they can't typically take the thermal shock of heating.
What happens is that the glass absorbs microwaves differently than the food being heated; if you're heating a liquid, the top of the glass above the liquid can stay cool while the portion touches the liquid heats up from conduction with the liquid. Not all glass can take that strain, and it breaks. (the opposite happens in the oven, the stuff above the liquid line heats while the part in contact stays cooler.
In theory, every piece of glassware that is rated for the oven should be safe for the microwave, but I'd be wary of colored glass, as they're typically colored using metal. (eg, I have blue glasses, which is often cobalt). Things that are neither oven save nor microwave safe include:
- Metal rimmed glasses
- Glass with air bubbles trapped in it
- Thin walled containers (anything less than a typical mug thickness)
One test for microwave-safe containers is to heat the glass in the microwave on high for a couple of minutes (possibly more if you have a low-wattage microwave), and see if it heats up. If it does, it's definitely not microwave safe. When I've done the test, I check every 30 seconds or so and stop if it's warm.
inserted: You may want to place another microwave-safe container with water in the microwave when testing (see comment from Ecnerwal). If you do this, you'll also need to increase the time, to let the test container absorb enough microwaves (as a good portion will be absorbed by the water)
... but that really only checks to see if it absorbs microwave radiation (which was a frequent problem when microwaves were new). It won't test for thermal shock, which may not happen the first time, but after a few uses.
These days, new products will be advertised as 'microwave safe' or possibly as not microwave safe, so if you're unsure, stick with the ones that claim to be safe.
As for speed issues -- if the container breaks and you have to clean up the mess (and can't eat the food, then yes. I don't know how much the vessel absorbing microwaves actually affects speed ... the part in contact with the food will conduct heat to the food (especially if it's liquid or relatively wet). You might be slowed down wondering if it's safe to eat after microwaving something in plastic, and finding the rim partially slumped. (which I have no idea if it's safe. When I was a grad student, I'd probably have eaten it. Now I'd likely pitch it)