You can use any time you like to make desirable prints.
Consistency is the real problem with short times, plus if you wish to hold back an area or burn an area, longer times will assist you to be more consistent.
I usually ensure that I expose for at least a three second exposure, this gives the lamp time to warm up and be consistent.
Using f8 will bring your times down to 5 - 8 seconds, nothing wrong with those times, as long as you are consistent.
You don't mention whether your enlarger is equipped with a colour head, if it is, you can make neutral density by adding equal amounts of all three colours. For instance:- adding 15 units of Cyan, Magenta and yellow is ½ a stop of neutral density, 30 units of each is 1 stop of neutral density.
C41 B&W film is usually thinner than many home developed normal B&W films, especially if you have under exposed and have slightly thinner than normal negatives for that process.
If you enlarger is the condenser version, then coupled with a thinnish negative, I can well believe the times you are doing to be about normal.
As for using a dimmer, I don't believe it would be too good an idea, unless you used the dimmer all of the time. My reasoning is that as you reduce the light output, your lamp changes colour, this in turn will change the colour of the light reaching your paper, which in turn changes the contrast.
This may mean that you will have lower contrast prints, forcing you to use a higher contrast filter.
If your enlarger is equipped with a Beseler lens, it would be at the worst, of quite reasonable quality. Generally the type of market that an enlarger is designed for, should give you an idea of the type of lens.
I'm assuming your enlarger is for the lower end of the market, schools, public darkrooms, amateur home phorographers and places like that. This would suggest that your enlarger and it's lens are of quite serviceable quality, learn how to make it sing, you should be able to surprise yourself with what you will be able to wring out of it.
You'll know when it's time to move on!
Mick.